You can have the OBi forward an incoming call on SP1 when it's not answered after a particular number of rings (not seconds) using:
Voice Services -> SP1 Service -> CallForwardOnNoAnswerEnable : (checked)
Voice Services -> SP1 Service -> CallForwardOnNoAnswerNumber : SP2(12341234567)
Voice Services -> SP1 Service -> CallForwardOnNoAnswerRingCount : 5
The Google Voice number you forward to cannot be the same Google Voice number you have configured on SP2, however, as calling your own Google Voice number is treated as a call to 'check voicemail'.
The CallerID of any call using Google Voice is the Google Voice number.