The master volume on most amps, keyboard or otherwise, is usually prior to the the output transformer used to drive the speakers, mainly in the primary circuit of the amp's final stages. The input volume controlled through the mixer is most often a small, 20K-ohm pot that dampens the signal, then passes it through the EQ circuit, through the Pan circuit, through a pair of blocking diodes, and finally through the main gain and outputted into the output transformer's primary coils. The secondary coils are what then sends the signal to the speakers--at least that the way my Peavey's, Crates and Polytone amps are wired. I'm not sure about some of the other brands, but I did run across a keyboard several years ago that had a positive ground, which was used in some systems in the early days of keyboards. You couldn't even connect this particular keyboard to an amp that had anything other than a floating ground without blowing a circuit breaker. I rigged a resistance mixer with blocking diodes for the guy and he was able to then plug into most PA's without encountering a problem. The only test to determine the safety of hooking a "Y" connector to a single foot-volume control would be to measure the electrical potential betewen both keyboards using either a voltmeter or an oscilloscope. He could get lucky and everything might be OK, but I would be very cautious about this one.
Gary
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)