One OBi 200 supports up to four different Internet Telephone Service Provider (ITSP) configurations. This means that you can, in your example, have SP3 configured to use a second Google Voice account.
All inbound calls to either Google Voice number will ring the attached telephone(s). You can manually set which of those 3 SPs is the default to use for outbound calls, and, if you want to call from a SP other than the default, you simply add **1, **2, **3 or **4 in front of the phone number to direct the call to SP1 -- SP4 respectively.
You don't need a OBi 202 unless you need two physically-separate analog telephone lines plugged into two separate telephones.