Re: One Hookah per flavour
Well, yes, it would be "better", but that's quite a lot of money/cleaning time/etc.
A good cleaning after a pungent flavor will do a hookah good, of course. Cleaning a hookah is like cleansing it's palate, so the next flavor or brand you smoke will come through full force.
The main problem with ghosting flavors is with hoses. Razan/Nammor/Grand hoses do not absorb flavors like leather hoses do, and are a very good investment.
Hoses, on the other hand, should be kept separate. As you mentioned, Double Apple is a very strong flavor and should have it's own hose if you enjoy smoking it often. I personally have a plain Desi Murli tobacco hose, I love the flavor of that tobacco and I want the complexity to come through each time I smoke it.