Back when I was on cable our provider would assign up to 3 real IP's for connected devices. If yours is doing something like that and both routers have an internet routable IP, that could work, but it's because you would have 2 real IP's.
Hit
http://www.google.com/search?q=what+is+my+ip from each server to see if they show the same external IP, or look at the WAN settings for both routers. If they match it won't work.
You need one Internet routable IP (nothing from section 3) per server.
http://tools.ietf.org/html/rfc1918