Co-transformation is useful for
Co-transformation detects simultaneous uptake and integration of two markers from same donor fragment, indicating they reside within length of transforming DNA. By scoring frequency of double transformants versus single, geneticists estimate proximity and linear order of three factors using two-point and three-point crosses. Higher co-transformation implies shorter separation. Beadle-Tatum era used auxotrophic markers to construct first bacterial maps before conjugation mapping. Because transformation does not require cell contact, it allows mapping closely linked genes that interrupted mating cannot resolve, resolving clusters like tryptophan operon organization.
Ref: Hartl & Ruvolo, Genetics, 6th ed., Chapter 6: Gene Mapping by Cotransformation; NCBI Bookshelf