Voice Services -> SPx Service -> X_InboundCallRoute : .......,{?:sp1(1aaaXXXXXXX)},.........
The syntax specifies which outgoing trunk is used for the forward leg of the call, in this case, sp1. So, no, SP2 is not used to send the call to GV2 Voicemail. This way, you do not pay for the outgoing calls. If you wanted to use sp2 (Callcentric setup on sp2, the syntax would be:
Voice Services -> SPx Service -> X_InboundCallRoute : .......,{?:sp2(1aaaXXXXXXX)},.........
Again, you can also put the first syntax in your inbound call route for sp2, so call coming in on your callcentric trunk (sp2) would go out on your GV1 trunk. This is assuming of course you have a callcentric DID (telephone number).