The present invention relates to an aqueous solution which contains a polymer of the monomers acrylic acid and methacrylic acid in a weight ratio from 1:1 to 1:10, to the production thereof and to the use of this solution as a base material for aqueous polymer dispersions. The water-in-oil emulsions (W/O emulsions) are used to prepare polymer dispersions. For this, aqueous monomer solutions or aqueous polymer solutions (e.g. a polymer dispersion) and organic media are fed to a polymerization reactor, usually under stirring, the aqueous solution being immiscible with the organic phase. The monomer droplets are thereby first completely or partially polymerized into polymer drops and finally polymerized into polymer drops surrounded by an organic phase. The polymer dispersion as end product forms. The polymer dispersion as end product is, in a first step, worked up by removing at least a part of the organic phase. Polymer particles containing the organic phase are thereby obtained, which particles are finally subjected to a coagulation step, wherein the polymer dispersion is heated, for example to temperatures in the range from 50 to 100° C., or else sheared, for example by using a rotor-stator machine, to a degree such that the organic phase is extracted from the polymer particles. The polymer dispersion of the invention contains preferably aqueous polymer particles. The aqueous polymer dispersion is usually used as a coating material, e.g. in paints, for coating applications. The performance properties of polymer dispersions (PD), which are important, in particular for their use in coatings, are, in particular, the stability and the viscosity (which is influenced by the molecular weight). To attain the desired performance properties, usually a compromise must be made between the polymer solids, the molecular weight of the polymer and the size of the particles in which the polymer is present. Polymer dispersions (PD) are usually stabilized with emulsifiers. Examples of commonly used emulsifiers are nonionic emulsifiers (e.g. polyethyleneglycol alkyl ethers, fatty acid partial glycerides, polyethylene glycol esters) and anionic emulsifiers (e.g. alkali or ammonium salts of carboxylic acids or sulphonic acids, esters of polyvalent alcohols and fatty acid partial glycerides). Depending on the monomer composition and the temperature, different emulsifiers are generally used in the production of polymer dispersions in water/oil emulsions (W/O emulsions). Polymer dispersions usually have high solids contents of from 60 to 90% by weight, based on the aqueous phase of the dispersion. A problem in the production of polymer dispersions with high solids contents is the poor heat stability of the dispersion. The stability of dispersions with high solids contents is poorer than that of dispersions with low solids contents. Examples of common dispersants which may be used for aqueous polymer dispersions are sodium alkylaryl sulphonates, diallyl sulphosuccinates, anionic carboxylic acids, such as acrylic acid, methacrylic acid, itaconic acid, or the salts thereof, for example sodium salts, or else anionic carboxylic acids with at least 12 carbon atoms, for example fatty acids or fatty acid esters, and nonionic emulsifiers, for example ethoxylated fatty alcohols. In order to stabilize the polymer dispersion and thus attain good performance properties, a broad palette of suitable emulsifiers is available to the person skilled in the art. A disadvantage of the use of emulsifiers is that it entails considerable extra costs and some of the emulsifiers which are used are ecologically problematic. It is known from U.S. Pat. No. 5,169,746 to stabilize polymer dispersions using ionic (cationic and/or anionic) copolymers. U.S. Pat. No. 5,750,788 discloses the use of polyacrylic acid and salts thereof as an emulsifier for acrylic polymer dispersions. EP-A 0 649 059 describes a method of preparing water-in-oil emulsions in which at least one water-soluble (meth)acrylic polymer is added to an oil-in-water emulsion comprising a water-insoluble (meth)acrylic polymer in an amount which is sufficient to render the water-in-oil emulsion stable. JP-A 2001 245 984 discloses a coating material with a polymer (1) prepared using ethyl acrylate and methyl methacrylate. EP-A 0 591 908 discloses a copolymer (A) which contains at least 15% by weight of units derived from (meth)acrylic acid and has an average molecular weight of less than about 50,000. The document also discloses the use of this copolymer in aqueous polymer dispersions as a protective colloid. The object of the present invention is to provide aqueous polymer dispersions which have better performance properties than the polymer dispersions of the prior art. We have found that this object is achieved by the aqueous polymer dispersions of the invention. The polymer dispersions of the invention contain polymers (A) and (B) which have different chemical compositions. The weight ratio of polymers (A) to (B) in the polymer dispersions of the invention may vary within wide ranges, in particular between 5:95 and 95:5. The aqueous polymer dispersion of the invention contains polymer particles (I) and polymer particles (II), the polymers (A) and (B) being contained in these two groups of particles in different proportions. The aqueous polymer dispersions of the invention contain polymers (A) and (B) which have different chemical compositions, the polymers (A) and (B) being free from a polyfunctional monomer selected from C2- to C20-alkenyl esters of acrylic or methacrylic acid or vinyl esters of aliphatic C2- to C20-carboxylic acids. Preferably, the polymers (A) and (B) have different monomer compositions. The polymers (A) and (B) preferably have a content of from 50 to 99.9% by weight of units derived from at least one of the monomers selected from acrylic acid, methacrylic acid and C1- to C4-alkyl esters of these carboxylic acids and a content of from 0.1 to 50% by weight of units derived from at least one other monomer selected from the other monomers. In this case, the polymers (A) and (B) preferably have a content of from 50 to 99.9% by weight of units derived from at least one of the monomers selected from acrylic acid, methacrylic acid and C1- to C4-alkyl esters of these carboxylic acids, in particular acrylic acid and/or methacrylic acid, and a content of from 0.1 to 50% by weight of units derived from at least one other monomer selected from the other monomers. The polymers (A) and (B) preferably contain at least one monomer selected from the group consisting of C2- to C4-alkyl esters of acrylic or methacrylic acid, vinyl esters of aliphatic C2- to C4-carboxylic acids and acrylamide and/or methacrylamide. In a preferred embodiment, polymer particle (I) contains from 1 to 50% by weight, based on the total weight of polymer particles (I), of polymer (A) and polymer (B) is present in polymer particle (II). Polymer particle (I) contains preferably from 20 to 90% by weight, particularly preferably from 50 to 90% by weight, based on the total weight of polymer particles (I). The polymers (A) and (B) may be partly or completely free from emulsifiers. Emulsifiers may be present in the aqueous polymer dispersions of the invention as additives. The use of these emulsifiers is preferred when polymers (A) and (B) have different chemical compositions, in particular when (I) and (II) polymer particles are present in the aqueous polymer dispersion of the invention. The emulsifiers are preferably added to the aqueous polymer dispersions in an amount of from 0.05 to 5% by weight, based on the total weight of the polymers (A) and (B), preferably from 0.05 to 2% by weight, particularly preferably from 0.05 to 0.5% by weight. Suitable emulsifiers are disclosed, for example, in “Emulsions and Emulsion Technology”, P. Lovell, J. Hewlitt, Ed., Lausen Publishers, New York 1992, pages 92 ff. The emulsifiers include, for example, nonionic emulsifiers, for example ethoxylated fatty alcohols, alkylphenols and polyoxyalkylene ethers. An example of a suitable emulsifier is polyoxyethylene (20) sorbitan monooleate (Tween® 20). It is also possible to use ionic emulsifiers, for example sodium lauryl sulphate, sodium alkylarylsulphates, sodium salts of alginates, for example sodium alginate, sodium acrylate, sodium methacrylate, sodium 2-acrylamido-2-methylpropanesulphonate (AMPS), and the potassium and ammonium salts thereof. It is also possible to use anionic or cationic emulsifiers with an amine function, for example dimethyldiallylammonium chloride, mono-/polyacylamines, for example a condensation product of acrylamide and/or methacrylamide with fatty acids, and also other cationic emulsifiers which are commonly used for water-in-oil emulsions, such as methyltrialkylammonium chlor