Delete the Google Voice service provider configuration on both OBi's via the OBiTALK dashboard.
Then, set them up again from scratch, again, from the OBiTALK dashboard. Before setting up each OBi, log into the Google Voice account you want to use, in one web browser tab, and then log into your OBiTALK dashboard in another browser tab. When you go through the procedure to add Google Voice, a window will pop up showing the GV account that is going to be used for that OBi. Make sure it's the correct one, then continue. After you configure one OBi, logout of GV in the GV browser tab, and log into the other GV account. Then, set up the other OBi.
Here are the full instructions:
http://www.obitalk.com/forum/index.php?topic=8560.msg56460#msg56460