I’ve done it before - I watched a Liverpool / Spurs game at White Hart Lane back in the day, and had to cheer when Spurs scored and act annoyed when Liverpool scored. It’s certainly do-able, but you wont be able to enjoy it anywhere near as much as if you could just celebrate as you like.

If you do openly cheer for United, you’re going to get the shit kicked out of you If west ham lose - you’ll probably just get insulted incessantly if they win.