A few times I ran mics with simple Y cables into my R4, both the Transparent Mod and Warm mod channels. I was curious to see how the different mods sounded (it's a subtle difference). One night I ran with the Y cables for set 1. Then on the second set, I removed the Y cables, plugged those mics into just into one pair of channels, and plugged something else into the other pair. When I listened, I was immediately struck... "wow those mics sound a lot better on set 2 than they did on set 1".
OK, that's one data point, maybe it doesn't mean much. And because it's a moded deck I have no idea with the input impedance is. But I guess I would say "do it, have fun, but take the results with a grain of salt" because maybe the split effected the resulting sound. I don't recommend doing that, then running out and spending $$$ on a new pre because of that perceived difference.
I have splitter snakes I use while multitracking, and no one seems to question the effects. This is the same as simple Y cables, x 24. I have FOH's encourage me to bring the splitter snake rather than using the direct outs. I read tales of FOH's refusing to let people use passive split snakes, insisting on tranny isolated, or active splits. At least in the clubs I working, no one seems concerned.