If the DETCHANS keyword is in the input file, dmtype2split will copy the value to the output. If the keyword isn't included in the input file, then it is missing in the output. This is not a problem for Sherpa users, but causes an error in XSpec.
Manually add the keyword to the file, either before or after running dmtype2split:
unix% dmlist acisf00459N004_pha2.fits cols | grep COUNTS 8 COUNTS count Int2(8192) 0:32767 -1 Counts array (a spectrum) unix% dmhedit infile=acisf00459N004_pha2.fits filelist="" \ operation=add key=DETCHANS value=8192