@elhobbs I tried the new version and it works, but it still starts without the sky and with crude graphics.
Instead I tried to start DualHexen.nds via hbmenu but sadly it gives two black screens and locked the console. Of course even with .argv file containing e.g. "DualHexen.nds -file hexDD.wad" it crashes.
Even with the old version of DualHexen launching it via HBmenu the former would crash. They seem to be incompatible. Is there perhaps an alternative homebrew launcher to try?
Side note: by DS mode I don't mean via slot1 (flashcart), but by setting it as an option in TwilightMenu++ by pressing Y on the Hexen icon: strangely enough you can set DS mode with DSi RAM at the same time...