Yes, there are enough skill points to split between mage and warrior skills, though of course you will have to be a little more selective than when playing one skill class (per character). Playing a warrior/archer I had unused skill points for both characters most of the game, and didn't have any problems from that.
For example, with warrior weapon expertise the first skill point into the damage bonus gives the greatest effect, with a slow increase after that (if you are not going to max it, level 1 is good enough). A couple levels of accuracy helps, but with stat and equipment bonuses as well, you shouldn't need to max it. Critical Hits and Deathblow can help, but have a low chance of being triggered unless you put a lot of skill points into them (I ignored them and didn't have any problems). Actually, the only weapon expertise section I put more than a couple skill points into was Reload Time in bow/crossbow expertise.
With the mage skills you would probably want to concentrate on the most effective spells, such as shaman magic (available starting in act 2).
You can unlearn skills in BD (at a cost of gold dependent on your character level) so that can free up skill points if you no longer need a particular spell.
I would suggest making the DK the battle mage. His armour class increases with character level, so you will not need as much strength for equipment requirements (depending on weapon choice). This would let you put more into intelligence. The math appendix of the
Beyond Divinity Strategy Guide contains a description of the primary stat effects and some suggestions on stat point distribution.
