And remember, two couples means four people. You can also use couple as a verb. You can couple an engine to a gearbox, for example.

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.

You can say a couple and two couples. You could also say "a pair of couples", but it sounds a little awkward. I would much rather prefer to say "two couples" than "a pair of couples"