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.

Aslong as you don’t show any colours, don’t celebrate united goals and don’t let anybody know your a united fan Sorry it’s not that culture

I can only suggest trying football ticket pad and then ilk sites for a ticket if you can afford it as a one off expense as a experience

Everyone does that at West Ham as the tickets are so easy to get