I actually stumbled upon this comment from a previous post: Midi channel "clash" - #15 by thetechnobear that says the two streams are merged. Clearly if no one chimes in here to tell me otherwise, it looks like my initial suspicion will have turned out to be true.
It’s not a huge obstacle to change the multitrack bank going from keyboard track recording to drum track recording, but also maybe it’ll motivate me to use the Pyramid itself more directly for drums instead of my drum machine. Also it looks like I won’t need to use a MIDI Merge box for my second keyboard/MIDI controller if there’s one implicitly baked into the Pyramid.