[SR-Users] Two GWs with the same IP

Greetings, I'm building some testing procedures for my Kamailio proxy. In order to do that i have a SipTester (Acting as Calling Number) from where i want to originate calls to Kamailio. After Kamailio processes the call, i want it to route it to SipTester again so it can act as Called Number. This should work with only one SipTester in one machine, so i created two SipTester GW's on Kamailio, belonging to different users and GW groups. Also, the two GWs use different Kamailio addresses to communicate with the proxy. However, in this combination, when routing i get this error " ERROR: dialog [dlg_handlers.c:666]: pre_match_parse(): bad request or missing CALLID/TO hdr :-/". If i change the destination or the origin of the INVITE the calls work without a problem. Does this happen because both GW's have the same address? Best Regards