Is the above possible?

I have just obtained two ATA's (by accident, bought two on ebay by mistake!). If it's possible I want to use something like asterix as an internal PBX on a server, and have two internal extension numbers. However, all I want externally is the one SIP number, coupled with a Gizmo IN number and Gizmo OUT line.

So - two internal POTS phones connected to ATA's, that can call each other using internal extensions. Both phones must ring on an incoming call, and either phone can call out.

Is this possible and if so how would you go about it?

Thanks,
Tom Eaton