Now we just need a way to use that shelf with the same account so I don’t get booted from my steam deck games just because I left something running on my PC and vice versa.

AFAIK, this is also a licensing issue. When Steam was launching, game publishers were concerned that people would simply share an account. So part of Steam’s licensing agreement is that the same account can’t have games (even different games) running on two machines at the same time. It’s specifically to prevent account sharing, because people would just share an account with their friends; Booting them out of their game every time their buddy boots something up is a pretty effective countermeasure.