Hello,
Kamailio can send a request anywhere you decide, the problem here is the FreeSwitch -- it will reject the re-INVITE if it didn't receive the initial INVITE.
The clean way here will be to transfer the call, so the first
freeswitch will transfer it to the one you want after re-INVITE.
You can add the new destination from Kamailio as an extra header
on re-INVITE. Alternative is to bridge from first freeswitch to
the second one, eventually with bypass media after re-invite.
A common use case is to differentiate between voice and fax tel numbers, then you can route from the initial invite based on the DID. Or have the freeswitch configured to handle both voice and fax calls.
Cheers,
Daniel