How can i assign two SIP trunks on a single server running UCOS CUCM 8.x with a specific dial plans for each trunks?

Hi

If you connect CUCM cluster to any Voice Service Provider via SIP Cisco recommends to use CUBE for that and for safety propose first of all. Also you could use any another SBC like AcmePacket or Metaswitch and etc. You can configure different dial plans for several SIP trunks on it. It's not so safe to connect CUCM to SP directly but it's possible in enterprise networks. For that you could use prefix and predot mechanism on route list level for different route-groups but again it's not recommended because of poor scalability and safety. 

Alexey