Well, very simple. An example – CIsco ATA 18x wih 2 FXS ports but one IP.
2 separate endpoints (each for 1 port) register in the gatekeeper with the same IP, but different aliases. How, you will ask, the gateekeeper does differentiate them?
Well, by adding the tcp port number to the IP.
the first will be:
x.x.x.x:1720 , the second
x.x.x.x:1721
your case is similar, I think.