My first choice was creatures, but you can make it any pair of types you like except planeswalker. No limits as to rarity or legendary-ness.

Could you clarify what you mean by "run counter to each other or seem like opposites"? When I think of opposites in Magic, I think of Black Knight and White Knight, Lifeforce and Deathgrip, Negate and Essence Scatter. Is that the kind of thing you're thinking of?

More like Melira, Sylvok Outcast and Phyrexian Vatmother. Or, for an example from creatures I created, Marthanos, Sage of Battle and Zerald the Covetous.
Would it be clearer if I said "Two cards that can hose each other, yet also work well together?"

OK, I think I understand a bit better.

It's a bit similar to your example, but the obvious answer to me was: Earth Maester and Sky Maester, similar to previous cards but paired this directly.

That is a tougher challenge than it looks. Mostly, because what it really wants you to do is to come up with an interaction in magic that hasn't happened before. I created Unparalleled Growth and Gruumsh Simpleton which brought forward an interaction that barely comes up, but I feel like I failed in some way since you could already do this with a couple of old Magic cards. I'll think on this one, and maybe I'll come up with something totally new later.

Good challenge. Just very hard.

I knew it was challenging when I made it, but I didn't think it would be hard enough that people wouldn't do it. I should probably get around to actually doing it myself.

Yeah, this turned out to be a bit harder than I'd realised. I had a go with Cardinal of the Ebon Hand / Cardinal of the Holy Nimbus, but I'm not sure it's a very good entry.

My own entires are Soulgate Demon and Vim.

Let's try Risen Knight

Nullus Troll Treason