I am currently running this type of setup in my house. I am only running one additional set of speakers in an upstairs living area. I am using an integrated amp with a long run of speaker cable that I threaded up through the basement floor and using the CDP's 2nd output (which I also have) and the tape out from my main system into the tape in on the integrated powering the upstairs speakers. This does mean that I cannot run my tape in right into the tapedeck in my main system for optimum recording quality, but I can always simply switch plugs if feeling compelled to do so and I am running the tape out of the second integrated back into the tape deck. I imagine the tape loop should be very close to passive with minimal degradation. I have thought about building or finding some kind of passive device that could split the tape out into two signals, one for the second system amp and one for the actual tapedeck, but haven't gone that far yet. You could also try using the pre-out from your main system and run it into an input on your second integrated, though I did not have any luck with this...the output was simply too weak. That may work in your system however and is probably worth a try.
I looked at your post again and see that I'm addressing this from a different angle as you want the second system to operate independently if desired. I suppose you can use the tape out from your second amp to accommodate that? Well, now if YOU aren't bored to tears, I'll just say good luck. Be sure to get the appropriate speaker cable for long runs. Numerous manufacturers design home theatre cabling for this purpose, although I've never considered what the effects on an amp may be, if any, with long runs of cabling like that. You may want to investigate that as well.