Re: dd split
I have a DD neck split. In fact, I have a DPDT on-off-on switch so that I can select either coil, or full 'bucker mode. I love it. It definitely gets that single-coil vibe when split. It has a nice, "airy", chimey, almost Strat sound when split.
If I was going to use a push/pull pot to do it, ie., only able to split to one coil or the other, I'd split the coil closest to the neck, in both positions. (neck or bridge.)
If, on the other hand, I was going to use a pp-pot to do both at the same time, I'd split to the inside coils. At least, thats what my ears prefer so far.