We have an iDR64 and a slaved iDR16. You are correct, the MixPad and OneMix apps do not control the slave channels at all, just the Master channels. I like just about everything about the iLive except this one little snag.
The iLive Editor controls both Mixracks just fine. At least, in our case it does but we are connecting Editor to a surface. I don’t know if it behaves any differently without a surface.
There’s a document on the A&H site called “Understanding Dual Rack Mode in iLive” that may answer your questions.
Mark Turner
T112/iDR64/iDR16/Dante
MixPad, OneMix
Mac Mini/Logic Pro 9