The phase cancellation was the kind of problem I was thinking of, especially if you're going to sum the split signal again, to go into a single amp. Even using two matching amps, it's tricky.
My splitter box is a little old thing from Boss, the J-5, 1 In, 4 Out, no batteries, no buffering, no active electronics at all! I've had it since the late 80's, and it still works; don't know why they stopped offering it. I used to place an MXR Micro Amp in front of it, for buffering, but any Boss pedal will work for that purpose.