Nighthawk AC2600 R7450 router. COD4, how do i use two xbox's and them not interfere with one another

So i Have two xbox ones hooked up directly to my router Via ethernet CAT 6 cables. I have one xbox as DMZ server and other one port Forwarded. If i am playing COD4 blackout and my GF turns on her xbox i get connection interrupted. Also i had my QOS for my xbox set on highest and i just keep hers at high and hers seems to run more smooth than mine and the QOS doesnt really seem to do much. Ill get 4,200 kbps bandwith and same with her. i feel like our bandwith should be higher and feel like the xbox's interfere with one another.. im stumped here. i have 30 mbps upload and  450 mbps download.