The destination port does not matter. Same reason why you are able to connect to multiple websites at the same time despite all of them being on port 80/443. What matters most is that the subnet must be different for each client.
Good insight as always from you.
I did some tests first using PIA where I was able to connect to two VPN servers, each server in its own subnet. Could connect with both clients using either the same port or different ports. It worked as you said it should.
Repeated the same test using two Astrill servers unable to connect to more than one server. When connected to the first server and then trying to connect to the second server I received an authentication error.
So it is a function of how the VPN provider runs their service and if they will allow multiple client/server connections.