I've been having fun recently layering one painting on top of another and seeing what happens! Experimenting with blend modes in both Photoshop and Painter I've found that Hard Light and Magic Combine (found only in Painter) seem to work best. Below are a few of my results, showing the 2 (or more) paintings that I combined:
this painting
plus this painting
equals this painting:
this painting
plus this painting
equals this painting:
this painting