Thanks for the quick response.
So there is no way to configure the kamailio out of the box for the scenario which I am trying to achieve. But do you think, that it is possible to program some simple kamailo module which will implement some really simple SIP client? (kamailio can create/modify SIP messages, it is able to receive and send them so I think that it is possible I am just not sure)
What I am trying to achieve is to encapsulate SIP communication between "SIP client A1" and "SIP client B1" eg. communication between these two endpoints will go through data link between "kamailio server A" and "kamailio server B".
I know that I will need to keep the database of registered users on "kamailio server A" as well as on "kamailio server B" and these "databases" will be synchronized by "kamailio main server" since both servers have this endpoint in common. I am thinking about using Subscribe/Notify for the purposes of updating servers' internal databases. That's why I want to create SIP session between "kamailio server A" and "kamailio server B" - probably for each call between two endpoints (eg. A1:B1, A2:B1) there will be SIP session between these two servers.
I've programmed simple kamailio module for receiving/sending SIP messages eg. I can parse the SIP messages before the kamailio will send them to UA. I am also able to send RTP packet from one instance to UA and I think that there won't be problem to send it to another instance of kamailio server.
What do you think?