I think making a combat bar and an "other" bar will mostly just confuse and frustrate players. Adding a multi-layered skill bar with an extra AP cost to spells for scrolling farther down will only lead to players dragging and dropping everything onto the top bar. I know that I would.
I disagree that combat in the game as it is now is tactical "enough". NWN2 isn't the best example either. Its magic mechanic is that you only have x slots per spell level per day... but its definition of a day is so flexible that you can switch, rest and recharge after every encounter. The restriction it imposes is that you can't change during an encounter, and you don't even want that restriction in D:OS.
Aramintai: It's fine that you want to be able to use all the skills at your disposal. If you want to swap your bow for a sword, it will cost you action points to do that. Spells are your weapons as well, and I think there should also be a cost for changing them in combat, even if it just a cheap 3 AP per swap cost.
The best interface solution I can think of to prevent having to drag and drop skills (which could definitely get clunky if you're doing 3 or more) is in the spellbook, click a skill to select it, and press the 1-8 button to replace the skill in that spot on the skillbar.
