Re the EA-ANT751 optimal distance, I have no idea. I would guess that they will couple if close, and that would be bad. Sufficient distance should be a few feet, rather than many feet. Two wavelengths (ca 12 feet) is way away, but longer cables means more cable loss. I would say just far enough so they aren't coupled. This may give you some insight -
http://www.ifwtech.co.uk/g3sek/stacking/stacking2.htm
(EDIT) Let me revise that - we are talking UHF here - typical wavelength at 500MHz RCH 19 is 2 feet. So separation by a few feet should be plenty.
Looking at the DB8e, they make the cable from the panel to the combiner as short as possible, and equal length. I'm sure the equal length is vital, and the cable length appears to be as short as possible and still allow the panels to pivot to 180 degrees opposed.
I'd expect that if equal lengths doesn't work with the EA-ANT751 combo, you should probably abandon that approach.