In SM, routing policies select a SIP entity as destination.
Correct, you cannot choose a particular entity link for a given SIP entity, but, consider:
Sip Entity named CMAppSequencingForSipPhones: 10.10.10.10
Sip Entity named CMIncomingCallsForPSTNSIPtrks:10.10.10.10
Build an entity link for the first CMAppSeq, port 5060
Build an entity link for the second CMPSTN, port 5062
You can then define a routing policy to use the 1st entity (and by association, its entity link) as destination, and a 2nd routing policy for the second.
Then build 2 SIP trunks in CM to accommodate. I've done this for someone with voicemail, SIP phones, AACC, etc all coming into CM, but wanting to have trunk group measurements in CM relative to the type of traffic such that each far end application had its own CM trunk group to pull reports from.