That's why I asked the question.

It's clear that you can be grappled by multiple foes. The PH is quite clear that you can't escape a grapple unless you beat
all your enemies in opposed grapple checks. But if you can be
pinned by multiple opponents, you get into weird cases where you're pinned by one guy, but merely grappled by another guy. Are you allowed to attack the guy who doesn't have you pinned? Heck, can you
pin that guy? I'd assume that "no" is the answer to these questions, since if one foe has you pinned, you're technically "held immobile." But the rules are muddy, and lack specificity on the point.
-Sagiro