cross network communication settings

[JiangDonglai98]

My question may be a little obvious, but I hope you can give me some advice. For cross netweork communication, by giving a suppositional example like: For party A, setup ./millionaire_prob_test -r 0 -a 172.19.10.151 -p 5051; For party B, setup ./millionaire_prob_test -r 1 -a 172.19.20.12 -p 5052. Can it works?
Here, what I'm wondering is how can two parties A and B know each other's socket server listening on which port?
In the example given above I assume that address and port defined after '-a' and '-p' both refer to what the other server are and is listening to. However, we donot set both party's server listening port, so how do party A and B know which port to listen on with the socket if we don't set it? Are the party A and get the same port to listen? If party A and B get the same port to listen, then the above example should be wrong, since they use different port. Am I right?