My opponent enters combat with a Pendulum Blade attached, they win the combat. Then they cast Ride the Wind to move to another battlefield, does Pendulum Blade add an additional +2 (+4 in total) or is each combat separate?

Yes — each move to a battlefield is its own trigger, so the unit ends that turn with both bonuses stacked (so +4 total if it moved to a battlefield twice), and winning or losing the earlier combat has nothing to do with it.

Pendulum Blade's text is "When I move to a battlefield, give me +2 might this turn." Ride the Wind moving the unit to a different battlefield is a second qualifying move, so the ability triggers again and puts another instance of "+2 Might this turn" on that unit — Might bonuses are additive and last for the rest of the turn; they aren't tied to a combat (FAQ #11904). Combat Cleanup only inserts "Heal all Units," which removes damage, not Might bonuses (Rule 468.1.a.1), so the first +2 carries over intact.

One thing to watch: the second +2 only applies once that triggered ability resolves off the chain, so if something kills the unit or you need to count Might mid-chain, resolve the trigger first.