Conformations of tethered chains enclosed in small spherical cavities were studied by computer simulations at high segment densities. The systems studied contain two types of compatible chains that mutually differ in length. They have been studied as models for cores of mixed multimolecular block copolymer micelles in selective solvents. Simulations were performed on a tetrahedral lattice using (i) mutually independent simultaneous self-avoiding growth of all chains, and (ii) modified equilibration algorithm similar to that of Siepmann and Frenkel [Siepmann, J. I.; Frenkel, D. Mol. Phys. 1992, 75, 59]. Only the geometric exclusion effect of segments was considered since it plays a dominant role in dense systems. It has been found that the systems studied are very disordered and the short chains in mixed systems decline more from the radial direction than the long chains. The distribution functions of pair distances of free ends of short and long chains in mixed systems significantly differ from each other. This fact is very important for interpretation of results of fluorometric studies on mixed systems. When a low fraction of tagged chains, which differ significantly in length from the nontagged chains, are added to the original system to get a fluorescently active micellar system, fluorometric measurements do not yield information on the original system, but information on conformations of added chains only.