Cant play 2 xbox's in one house

So basically I have a few friends over and we are trying to play some COD MW3 and we all have OPEN NAT types all able to get online but when we try to party it will kick us all out. UPNP is enabled. It is not the modem or routers problem as I have gone out and bought 2 different modems and routers with the same stupid results Motorola sbg6850. ive tried to do all of this stupid Port triggering or port forwarding but still same results people try to say that it is impossible to have 2 xbox's hooked up in the same house but ive done it at like 15 peoples houses no problem hooked up 5 xboxs at one point no issues. i need help this is getting rediculous i talk to geeksquad they wanna charge me 89.99 for their service plus 500.00$ to come out this weekend, just to freaking play a xbox......... everyother place i talk to they wanna charge me 50$ to do the same damn thing that ive done already probably over 100 times now and they wont credit my money back if it doesnt work. so if anyone knows what to do I have a Arris tm702g/ctn (comcast Modem) and a Belkin N+ F5D8235-4 wireless router..