I have used such a scheme...
...with my vintage Pioneer integrated...one leg of the "y" to feed my Citation power amp for my mains, the other thru an ambience processor and back into the integrated's "power-ins" driving the rears...There is a school of thought that "y"s can cause some adverse interaction(as offered by JR). I must say I tend to agree with him(fancy that). I personally never encountered any problem, but that was with my gear. There is always the possibility others might have different results.
Unfortunately, other outputs such as "tape out" or "tape mon" are useless in this particular app since(as I'm sure you are aware) their output is at a fixed line level and the volume and tone controls do not affect that signal...although, if I recall correctly, there can even be exceptions to that rule.
After a fairly long hiatus from the hardware(upgrade-itis) side of the hobby, I have found some of the newer stuff(while lacking what I consider essential switching options) do have the facilities for a "zone 2" with which either the same source or a SAP can be played. A newer receiver would simplify matters...but that might not be an option for reasons outlined above. Some newer "pre-"s also have multi usable outputs...they tend to be the "pricier" higher-end units, however.
jimHJJ(..."y"s should be OK...)