Thanks for all the reassurance everyone. The 2nd PAP2T arrived today, and installation was relatively painless. I first disabled my existing port forwards, and verified my original line still worked (like everyone said it would). Plugged the 2nd ATA in, and the only problem I encountered was calls between the PAP2Ts would give busies- a quick search indicated ATAs behind the same router needed to be on the same server, so a quick call to support and the new account was moved to the server I was already on. Problem solved. And YMMV, but with the latest DD-WRT build, leaving the router's firewall active didn't cause any problems.

So get us out of The Planet, and let's move forward!