Why is it essential that the same restriction enzyme be used to cleave (cut) the dna of both organisms used to create a transgenic organism?

Restriction enzymes (endonucleases) identifies and cuts the same (palindromic) sequence in both DNA and vector because when two different DNA molecules are cut with the same restriction enzyme, both will produce fragments with the same complementary sticky ends making it possible for DNA ligase to join it together and form a recombinant DNA molecule.