I bought a second PA on the steam sale so I could easily test two-player scenarios (like how resource sharing works) My original key was straight from Uber Would it be useful to try and tie it to my existing Uber account, or would that just complicate things? What should I do/not-do when registering to associate it (or make sure it doesn't)
You can't really link multiple game keys to a single account. Way too many ways people could do that accidentally when they really wanted to give the key to someone else. You can make a second account in this situation. While we normally don't encourage that, for what you're trying to do - and frankly a number of mod developers may do something similar, it's the only way to handle it. While it would be possible to allow for multiple activations on a single account, the use cases for that are very limited, and have a ton of places where it could cause undesirable behavior. It's simpler to just allow multiple accounts when someone has a legitimate need for multiple accounts.
For future reference of anyone else: you need a distinct email address. (Fortunately I have wildcard domains)