It sounds like you need a better understanding of how OBi devices work with Google Voice.
You can set up two different Google Voice accounts on one OBi device -- for example, GV#1 on SP1, and GV#2 on SP2.
You can select which number is used as the primary line for outbound calls, and which lines ring which of the two different telephone jacks, line 1 and line 2, on your OBi. To do this, just log into the OBiTALK portal, and set it up as you wish.
Assuming that SP1 is set as the primary line for outbound calls, then, to make a call using SP2, enter **2 before the phone number.
You don't need any of the fancy stuff as mentioned by Irosenman.