[color:"orange"]Is the human as a mage and deathknight as a warrior viable in hardcore difficulty?[/color]
I have not played a mage character at all, so can not comment on this. I don't recall anyone posting about any particular problems with mages in hardcore difficulty.
The more powerful and mana efficient spells start to become available in act 2, so for the start of the game you may want to make sure both characters have at least some fighting abilities. Skills can be unlearned, and a mage is going to need some strength and agility for equipment requirements, etc.
The math appendix of the Beyond Divinity Strategy Guide
contains a description of the effects of the primary stats, as well as some general suggestions on stat point distribution.
[color:"orange"]The human is closer to the main character so I want him to be my mage, though I know deathknight isn't necessarily the best melee character.[/color]
The DK works fine as a melee character. However, since the DK's 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.
I had no problems with a melee warrior DK and crossbow archer hero in tactical difficulty. Playing again, I would make the hero the warrior, and the DK a bow archer, so he could concentrate more on agility; with my hero archer, having to meet strength and agility requirements for armour and crossbows in addition to keeping constitution, speed and survival up enough was sometimes difficult.