Just to be clear, you have the VOIPo setup on another ATA (possibly a locked device) on your LAN and you have the credentials for that same VOIPo account to set up the same account on the OBi SP2, which would be used for E911? Does VOIPo allows two or more devices to register to the same account?
If that is the case, could you set up the VOIPo account on one of the Voice Gateway accounts using the supplied credentials? This will only work if VOIPo allows outgoing calls without registration and require a SIP account on SP2 [ or what I commonly refer to as a pseudo SIP account...127.0.0.1 ]
The other option is to do some form of IP dialing from the OBi ATA to the other VOIPo device. I had this setup where I could direct IP dial E911 calls from one SIP device (SPA3102) to another ATA on my LAN. I'm not sure how to set it up on my OBi110, but someone might be able to point you in the right direct.
Does VOIPo have sub-accounts? Does the Sub-Accounts have the same E911 capabilities of the main account? In that case you could set up the sub-account on SP2 (registered) and using Ianobi suggestion.