green arrow/black canery
spiderman/black cat
spiderman/human torch
spiderman/daredevil
cyclops/jean grey
hawkeye(clint)/Mockingbird
wasp(jan)/hank pym(any one of his idenities would do)
Captain America(steve)/Bucky Captain America
Captain America (steve)/falcon

DC: Starfire & Blackfire

Marvel: Mr. Fantastic & Dr. Doom

It would been funny to see how well arch-enemies would work out having to team up as duo figures in the way that Thor & Loki did.