I haven't read it enough times to think I understand it but ....
Up to 4 terminals may be specified in the list. The listed terminals will be called/rung by OBi simultaneously; we refer to this operation as forking the call. A terminal may be a trunk or an endpoint.
Instead of just forwarding the call perhaps the Obi can ring the phone port and ring another "terminal", simultaneously.
And after thinking about it, the Obi can't forward a call out sp2 to the same GV number that sp2 uses. Because the outgoing forwarded call will carry the sp2 GV caller id which will see the incoming call as it's own number and therefore the call will be answered by the GV attendant.
It can however forward the call back out on sp1 to the second GV number. It would probably be best not to have the second GV account set up as sp2, to eliminated a loop, unless the second GV account is only set to place calls and not receive them on the Obi.