It sounds to me like you're making the assumption that because you have multiple IP-enabled devices that they should all be able to talk to each other. It's understandable that you'd make that assumption, but unfortunately networking goes far deeper than that.
IP is the underlying protocol -- what other protocol (or 'language') the various devices speak sits on top of that. I'm guessing that your receivers are DLNA-compliant, which today is a fairly common media networking protocol. Unfortunately, the original Squeezeboxes came out before DLNA existed, so they rely on their own SqueezeServer/SlimServer protocol. This is why IcemanDallas says that you can't transmit to a non-Squeezebox device -- nothing else out there speaks the Squeeze language.
Now, if you were to set up a dedicated music server (which isn't a bad idea if you're looking to have multiple Touch/receiver setups), it's fairly trivial to have the server speak both the SqueezeServer language and DLNA at the same time. Vortexboxes
are quite nice for just this sort of thing. The nice thing with these is that you can purchase one that's ready to go, or you can build your own. Or there are a number of other roll-your-own ways you can go if you don't mind getting your hands dirty.