The process of adding complementary nucleotide extensions to different DNA molecules, e.g., dG (deoxyguanosine) to the 3´-hydroxyl ends of one DNA molecule and dC (deoxycytidine) to the 3´-hydroxyl ends of another DNA molecule to facilitate, after mixing, the joining of the two DNA molecules by base pairing between the complementary extensions. Also called dG - dC tailing, dA - dT tailing.