If something truly needs Port Forwarding, it is next to impossible to forward traffic to two internal hosts on the same port.
Port forwarding is a rule that says, "forward all traffic on this port to this IP if the connection is unsolicited".
Multiple computers can use the same application at the same time if the app connections originate from within your LAN because usually the source ports used to originate the connection are random, and therefore unique on both computers. Your SOHO router keeps track of which computer is using which source port, and when traffic comes back in, it knows by the different destination port which traffic goes to which computer.
For example, both my wife and I are currently using MSN Messenger at the same time. We're connected to the same server at the same destination port. Her SOURCE port is 61871, but my source port is 2210. That's how her IM's go to her, and mine go to me.
However, the port on our end is the same as a destination port on an incoming connection request, which is what port forwarding is used for.
IE, if both my wife and I wanted to host a web server at the same time on port 80, how on earth could the router know which IP it should send each incoming request.
Even more complicated - what if traffic is coming from the same IP for both our web servers? How would the router know which internal machine to send the traffic to?