BleepingComputer.com: 2 xbox 360s 1 computer

hello, i used to not have a problem running 2 xbox 360s at the same time through my router, but recently had went to comodo firewall version 3 now one or sometime both fail the nat setting on my xbox dashboards. i had no trouble using both of them when i was using windows firewall. i was told awhile back about static ip addresses for both of them but i can not figure out how to set both of them with 2 different addresses and have them both use the same ports. i tried linksys tech support and microsoft they say you cant have 2 xboxs running at the same time, but i know i can because for the last 4 years i have been. so what i am basically asking can somebody please explain to me in moron terms step by step. what all i need to set and allow? i have windows xp service pack 3, comodo firewall, linksys router wrt300n router wired, ethernet cables, motorola modem, 2 differnet xboxs. there are several people that have given answers on different sites but none of them i can get to work because they are all for 1 xbox. please help if you can sorry for confusing question. remember please write down directions like you talking to somebody with no brain.