If you already have the HDB91x, put it up, see what you get... then decide if you need to change plans.
I would opt for an Antennas Direct DB8e, which can be adjusted for maximum forward gain (narrow beam width) or varying amounts of forward beam width (lower peak forward gain). I would also consider adding an Antennacraft Y5713 pointed at real CH-8.
Combing two UHF antennas requires tuned passive filters or active electronics... (your own private CATV head-end, $$ to $$$$) Or skip combining and use an A/B switch or separate tuner,
http://forum.tvfool.com/showthread.php?t=2882