I saw this a while ago, but just seeing the thread again, it makes me wonder:What do you mean by "serious"?Of course in rock and roll or American improvised music, that's standard.Numerous performers of American improvised art music, including jazz, have done this: Jimmy McGriff with the acoustic piano and the Hammond organ. Bill Evans with the acoustic piano and the Rhodes piano. Herbie Hancock, child prodigy of legit music with several keyboards at once. I don't know why someone would do two acoustic pianos at the same time, but one way I like my setup at home is to have a single-manual Hammond on top of a Rhodes piano, at a right angle to an 88-graded action Yamaha piano, from which to play the lower manual of the Hammond. It doesn't make much sense for playing either pianos/Rhodes nor Hammond, but it's flexible.