You would have to use different Ports, say you on IP:83 , your other computer on IP:84.
You cant use both HFS on the same port, its not possible i think.
For Example, the External IP will connect on port 83 for one computer and port 84 for the other. If they are both on port 83 the connection will hit the router and well confusion ensues and it will just go to the easiest link... this is my little crazy theory, of course rejetto will know the definite answer, just put both on different ports and u will have no problems i think, have you tried this?