I have a XBox 360 (a UPnP A/V Media Render) which is connected to one of my
two networks at home. The XBox has the ability to play content from a UPnP
Media Server. It uses UPnP and Multicast to discover the Media Server. Is
it possible for the two routers to share the same Multicast Group so the
XBox can discover the content?

The XBox's default gateway is the gateway to the other network.

Cable Modem
Router 1 ## LAN: /24 # (Bedroom)
---- ----
/ \
Media Server Router 2 ## WAN: /24 # LAN: /24 #
(Living Room)
Xbox 360