Your question is not clear. A binary vector usually contains two antibiotic resistance genes already. One is for plant selection (on T-DNA fragment) and another one is for bacterial selection (on backbone).

Are you talking about different plant selection markers on each binary vector? Then, so you can select a transgenic plant containing both T-DNAs (from the two binary vectors).

How do you do your co-transformation?

For selection of the transformants having both the T DNA.