There are several options I can see to start from:
1. A rock/paper/scissors type board. The classic example is a dreadfully boring Militia/Library/Council Room BM game with Baker at the bottom of the Black Market Deck (with all all the other horrible cards above it, basically each player can open 5/3 but Bm is a horrid play and Baker unlikely to be drawn nicely). So if you both open 5/3, P1 can take any of the three cards listed ... and P2 can counter nicely. E.g. P1 buys Militia then P2 buys Lib. If P1 buys Cr, then P2 buys Militia. If P1 buys Lib, then P2 buys Cr. In theory, you don't need the wonky stuff with the Baker coin, but the point is easier to see without having concerns with the price point. Another case would be something like Young Witch/Bane/Silver. With a decent enough bane you can easily have opening Yw loses to bane which loses to silver which loses to Yw.
2. Reactions. Say there are decent attacks out (like Oracle, Militia, and Noble Brigand) and you have decent options in reactions like Beggar and Watchtower. Knowing which attack the other guy is committed to for the first few hands is an advantage if there are near equal strategies with the reactions (E.g. Watchtower is a lot better as dead draw against Militia, while Beggar is not as hot against Noble Brigand).
3. Embargo. Mostly this is for potion cards. If P1 commits to a potion buy for something he needs to spam on the potion side - like Apothecary, Alchemist, Scrying Pool, or Vineyards, you might be in a better position if you can use embargo to tank that and then follow a marginally less bad strategy.
4. Enabled strategies. Like with reactions, some setups key off player interaction. For instance, Embassy & Mint make for a first mover problem. Mint with a free silver is pretty good, Mint without a Silver is pretty terrible. If you P1 with 5, you have to play the odds on P2 getting Mint as well; if you are P2 you can safely open Embassy without worrying about easy Minting. Marauder/Death Cart can also be a bit of a thing here.
5. Higher player counts with limiting cards. P1 tips his strategy, P2 can then avoid competing for cards and take the 2nd best strategy. This forces P3 to fight P1 (possibly moving both's odds below P2) or to take the 3rd best strategy (high odds of not winning). If P1 opens Remake/Squire to go for a Remake/Lib/Sqr/Spy/Jester run then P2 can opt instead for a weaker option (like B-crat/Silk Road) hoping that fighting for Sqr will limit P1 and P3. Or, even more simply something like opening Yw (horrid bane) for P1 and P2 going for Beggar/Feoda - P3 either goes Yw (nerfing P1 and powering up P2's Beggars) or mostly concedes the game to P1.