There is no point in deliberately handicapping the DK. Giving a DK warrior a high intelligence will not make any difference.
With BD's stat system, there is a great deal of flexibility in creating viable character builds. Some people have played using two melee warriors the entire game. The math appendix in the Beyond Divinity Strategy Guide
describes (among other things) the stat effects, and gives some general advice on stat point distribution.
For the majority of the first act there are lots of skeletons around, which are more vulnerable to crushing damage. Until I reached the citadel (late in the first act) I had my hero use crushing weapons (a club or 2 handed walking stick) most of the time and a bow for tougher fights, then I switched to archery full time. My DK continued to use slashing damage (1 or 2 handed swords).
The DK worked fine as a melee character, but I would do it differently playing again. Since his armour class goes up with his character level, he will not need as much strength for armour requirements, and can concentrate more on intelligence (for a mage) or agility (for a bow archer). Having the DK use a strength based weapon negates this. Since the human will need to meet strength requirements for better armour anyway, this character is slightly better suited to be a melee warrior. This is a relatively minor difference, though, so a human mage / DK warrior combination is just as feasible, and that difference doesn't really matter much for two warriors.
I would say just be a little conservative with your skill points, and you will have a fair bit of flexibility in developing your characters. You can also unlearn skills, at a cost in gold proportional to the square of your character level.
Most weapon skills you can get a good effect with just a skill point or two. For my archer I maxed Reload Time (which gives a direct Speed boost), but for the rest I just put a point in damage and a couple in accuracy.
The goal of your characters is to undo the soul forging, so I don't think is is much of a spoiler to say that you will succeed at that. Consequently, the DK will not be with you in the after-game battle fields.
If you plan to keep playing after the main game, you should favour your main character with permanent potions and other bonuses, rather than the DK. Other than that, it doesn't matter much who you choose to give bonuses or equipment, etc to.
There is a point where you are about to return to Rivellon. You can strip your DK of his inventory, then (if you have enough gold) have him unlearn all of his skills and learn some summoning doll upgrade skills. The act 1 doll can equip a crossbow, so even if it can not really be upgraded to be as strong as a main character, it could still provide support in the BFs.