[Serusers] SIP Transaction and avpops

Hi All, I have the following scenario: There are 2 SER instants: Ser-1 and Ser-2 running on 2 machines. All the two are responsible for the domain example.com. Ser-1 is for outgoing calls and Ser-2 is for incoming calls. Alice(a)example.com wants to call Bob(a)example.com, so she sends an INVITE bob(a)example.com to Ser-1. Ser-1 forwards the call to Ser-2. Ser-2 tries to locate Bob but Bob's softphone is not registered. Fortunately, example.com has implemented "call forwarding on unavailable" service and Bob is using this. He wants to forward all his calls to Bob(a)somewhere.com, so Ser-2 replaces the URI of the INVITE by bob.somewhere.com and forward the call back to Ser-1 (as Ser-1 is responsible for all outgoing calls). My question is would it be possible that Ser-1 thinks that the 2 INVITEs (INVITE bob.example.com and INVITE bob.somewhere.com) passing through it belong to the same transaction? (Because they have the same CALL-ID, Fromtag and Totag). If so, How can I do to make them become 2 different transactions? Thank you very much for your help, Minh