The story of GEOS is quote interesting too. From 8bit OS for c64 in mid 80ies to powering Nokia Communicator and a bunch of random Japanese products in mid/2nd half 90ies.
DS software was specifically designed to use screens as separate units (for example, actual game video on top, extra data on bottom), not as a unified screen, which helped.
There were other dual screen phones other than the Axon, off the top of my head I can remember NEC Medias W and Kyocera Echo (which was the first Android dual screen phone iirc). There was also a Sony tablet with dual screens, Sony Tablet P.