It depends on how you're using "couple" for how you can phrase it. If you're talking about a "couple" as in a romantic partnership, you could say "those two are a couple" or "I saw some couples over there" and that kind of thing. If you're using "couple" to count, you'd say "I have a couple" or "there are a couple" and the like. "Two couples" is only correct if you're using couple as a noun as in the romantic relationship ie "there are two couples by the door." I've never heard anyone say "a pair of couples." It's sort of awkward sounding, even if you're using couples as a noun.