I once had a friend of a friend who checked in online, printed the boarding pass, saved the boarding pass as a PDF, opened it in Acrobat, changed the name and seat number, and printed a second one.

I'm sure it's highly illegal and probably classifies you as a terrorist or something.

When I needed to do this, I just asked nicely at the ticket counter when my wife checked in and showed them my 1K card and they gave me a gate pass. But who knows what you'll get - United employees can be so unpredictable.