dialtone,
I cannot think of a solution for your case, since both SP1 and SP2 are configured to use GV protocol.
But suppose you have SP2 still using a SIP ITSP (SIP Protocol), then what you want can be done for
SP1 (GV). That is, call forward incoming calls on SP1 to a SIP URL. The call forward destination number would be something like this: sp2(12345@xyz.com).
The reason is that the OBi is relying on at least one SIP trunk to perform SIP transactions.