@ m8chorOf course, you cannot do that (forward all data on the SAME port to DIFFERENT IP's).
What you need to do, is change the game hosting port (ie. 6112) for one of the two computers you want to host from in the "user.setting file" under the "GameHostPort" entry.
XP
C:\Documents and Settings\user\Local Settings\Application Data\Ironclad Games\Sins of a Solar Empire\Setting
Vista
C:\Users\yourname\AppData\Local\Ironclad Games\Sins of a Solar Empire\Setting
For example computer one would host on 6112 and computer two would host on 6113. Then forward port 6113 from the router to computer two.
You will still only be able to forward the ICO port (6000) to one computer at a time.
thanks,
the Monk