I wouldnt do the same muscles in a row, but thats me. I do flat bench and then do 2 sets of diff muscles that have close to nothing to do with bench and then hit the DB bench. I think of it as not going at the lift with max potential, cuz ur already a lil weak it will just make it harder... Kind of like a ladder u can do one rep at 85 lbs curl like cake but from a ladder starting at 50 then add 5lbs all the way to 85 that 85 would SUCK!
EDIT*** if your talking like 3 sets of 135 lbs with 10 reps on the bench you do those straight up dont interfere with those i think..