Janet's harmonies (and vocal arrangements for the most part) have been consistently complex from the days of CONTROL. I think in that regard, she tends to be influenced by the likes of Stevie Wonder and Prince.

Madonna's harmonies tend to be very straightforward and simple. She sometimes has 2 voices harmonizing (1 melody, 1 harmony) and then brings in a third on small parts in the choruses or at the end of the song for climax purposes (some examples that spring to mind: SECRET, FROZEN). But she almost never does anything more complex than 3 part harmony. It's never really been her thing.