Let's separate channels from speakers. Splitting the same channel to two speakers doesn't give you stereo.I'm thinking, if I just split those two channels for 2 sets of speakers, I could achieve that without having a 9.1 receiver.
2 speakers above you will let you hear left-vs-right separation overhead. 4 speakers above you will let you hear left-vs-right AND front-vs-back movement overhead. So there is a definite advantage to 4 heights.
However, if you're splitting the same 2 height channels between 4 height speakers, then you won't hear any front/back movement (both pairs of height speakers are playing the same info).