I am hoping to run kamailio in front of these instances and route sip and rtp traffic to the correct asterisk instance.

So in this use case asterisk instances will be used as application servers and kamailio will be used as a sip/rtpproxy with nathelper

So my question is what would be the best way and using which modules can i route the requests for customerone.whatever.com to the first asterisk instance and customertwo.whatever.com to the second.

is this achieved with the dispatcher module? and if so, where can it be defined that requests for cusomertone.whatever.com is routed to the first asterisk instance.