The issue here isn't the need for two routers (remember your residential modem can only give out one external IP) As such you only have a single pipe (that one external IP) feeding your whole internal network.. Even if the router was clever enough to multistream the relevant ports (most residential routers will not, no idea though about tomato (or equiv) and match them to the internal IP's you're still switiching from one internal IP to the other which would in essence slow down both XBOX's probably to a point of extreme lag. This would probably also be the case even if xbox had alternate ports..
The ONLY sure fire way of doing it is to gain another external IP (either an additional link (ADSL or VMB) or to swap your existing 50Mb VM for a VMB (5 IP's on 10Mb but only for ex-ntl) or go straight to an adsl supplier that will do you a block of 8 (5 usable) IPs
Also please do not start up any more threads on the same problem
