I don't use the OBiTALK Web Portal, so I can't help you there. What you need to end up with in the OBi is:
Service Providers -> ITSP Profile A -> General -> Name : Google Voice
Service Providers -> ITSP Profile A -> General -> SignalingProtocol : Google Voice
Google Voice #1 on SP1:
Voice Services -> SP1 Service -> AuthUserName : (Google Voice #1 username)
Voice Services -> SP1 Service -> AuthPassword : (Google Voice #1 password)
Voice Services -> SP1 Service -> X_SkipCallScreening : (checked)
Google Voice #2 on SP2:
Voice Services -> SP2 Service -> AuthUserName : (Google Voice #2 username)
Voice Services -> SP2 Service -> AuthPassword : (Google Voice #2 password)
Voice Services -> SP2 Service -> X_SkipCallScreening : (checked)
Physical Interfaces -> PHONE Port -> PrimaryLine : SP1 Service or SP2 Service
You choose a particular trunk (SP1 Service, SP2 Service, OBiTALK Service, or LINE Port) as your PrimaryLine.
To use an alternate trunk instead, you prefix the number with:
**1 -> SP1 Service
**2 -> SP2 Service
**8 -> LINE Port
**9 -> OBiTALK Service
If you configure the OBi itself instead of using the OBiTALK Web Portal, you must also set:
System Management -> Auto Provisioning -> ITSP Provisioning -> Method : Disabled
System Management -> Auto Provisioning -> OBiTALK Provisioning -> Method : Disabled