Google Voice is a service, in the same way as Gmail, Calendar, Docs, etc. All Google services are tied to a Google account (Gmail address and password). One Google account can have a maximum of two inbound Google Voice phone numbers. It has nothing to do with "home" or "work" status, which only impacts linked/forwarding phone number use.
However, you can't treat two GV numbers separately when they are assigned to the same GV configuration on that account. All inbound calls to either number will forward to the same linked phone numbers (and to the same Google Chat configuration on one OBi SPx). If unanswered or busy, all inbound calls to either GV number will end up in the same GV voicemail box.
If you want to use the second number independently, then you'll need to port it into a different Google/Gmail/GV account than the account that holds the first number. Wait a week after porting the land line to T-Mobile, before subsequently porting that number to Google Voice. Once done, you could configure GV #1 on SP1 and GV#2 on SP2 on the OBi, and you can direct those calls to the two independent phone 1 and phone 2 jacks on the OBi.