Depending on what you're trying to do, you could just use different port ranges for each machine's applications. Like if you want to run game servers on each one, then run one on like port 2001 to 2010 and the other on 2011 to 2020, or the same for whatever else, if you're allowing inbound connections. Just on the other end you'd have to have the incoming client specify what port to use, although for a game server most current games already have the port visible and used from their server browser.
If you've run out of port forwarding (no way to increase them past 10), you could switch to ranges instead of individual ports, although that means forwarding ports you aren't actually using. So if you want to forward ports 80, 21 and 23, you can make a single line to forward 21 to 80 to that IP. But you still can't forward one port to more than one IP.
High-end routers can forward based on the source too, and other variables, so you could forward the same port to different IPs, but you can't afford those. 🙂 And of course you can still only have a port open once at a time on the WAN, but port remapping frees them up pretty quickly if it's common like FTP or HTTP.
