The way I see it, a good DW combo would have you shooting your enemy non-stop. Therefore, you would want guns with vastly different reload times and firing rates. Here is a list of combos I would recommend.
BPR=Brute Plasma Rifle
PP(C)= PP (charged)
These one should be fired so that at least one gun is firing at any given time (Begin shooting with one, wat a second, begin shooting with the other)
PP+PP (NOT SUGGESTED)