Methods and compositions are provided for introducing an expression cassette into a cell. In the subject methods, a population of at least two distinct adeno-associated viral particles is provided, where each distinct type of viral particle in the population comprises a different portion of the expression cassette to be introduced into the cell. The target cell is contacted with population of adeno-associated viral vectors under conditions sufficient to produce a hetero-concatemer in the cell, where the hetero-concatemer includes a functional expression cassette having an intron that includes an ITR sequence. Also provided by the subject invention are vector preparations for practicing the subject methods as well as kits for use in producing the vectors employed in the subject methods. The subject methods find use in a variety of different gene transfer applications, including both in vivo and in vitro gene transfer applications, and are particularly suited for use in the transfer of long genes.