previous Y connector discussion
Klipsch forum discussion of Y connectors from preamp
AVS Y splitter from preamp discussion
AV forums discussion on Y splitters from preamp
I feel like a kid with a new toy with this url thing that @Dill showed me how to use!
Anyway, @griz, I don't know enough about this stuff to say you can or cannot safely do this, but I do know that back in my days of HT I called Cary Audio and asked the tech guy whether it would be okay to split the signal from pre with a 'Y' splitter, and the tech guy was okay with it. What I was doing is not the same as what you are doing, however, I wanted to try (and did actually do this for a while) splitting the center channel signal from my HT preamp and sending 1/2 to one side of my Cary tube amp, and then sending the other half to the other side of that same Cary amp, and then basically using that to biamp (so to speak) my center speaker.
Which is not what you are doing, as you are wanting to send one signal to two different amps, and do that two times (signal split to main amp L RCA in and also to L sub RCA in and then the same for the R side).
Maybe post that question on amps and preamps forum and maybe @atmasphere may see it and provide an answer.