Google Voice is a service, just like Gmail, Calendar, Contacts, etc. All Google services are assigned to a Google account (Gmail address and password). So, you have two different Google accounts, and each one has its own Google Voice number. One OBi 202 can be connected to up to four different Google/Gmail accounts.
All you need to do is to sign into the correct Gmail account that has the number you want to use on SP2, then press Ctrl-T to open a new browser tab. Click SP2 to begin the procedure to set up Google Voice. When it gets to the step where it asks you to approve it accessing that account's Google Voice service, look closely at the Gmail address(es) displayed, and make sure you select the correct one that holds the second number.
This is separate and unrelated to the fact that you use your Google sign-in as your OBiTALK sign-in.