When I was first playing NWN I really wanted a Fighter(10)/Rouge(10)/Blackguard(10)/Assassin(10), but it turns out that the game only allowed me to have three classes at the same time. That with the level cap pretty much destroyed my dreams of godhood. Pretty much being the key word in the sentence.
The only problem the way I found was that you couldn't use some items.
What you need to do is, first get yourself a Leto character modifying tool.
Make a backup save of your character and start TWO Leto's. Open the .bic file in one and minimize it. Then open your character the usual way and use the Class wizard to remove one of your classes. Save and close it. DON'T touch the other one!
Start the game and launch the save. You'll notice that your character now doesn't have the same feats and skills. Save and exit.
Open another Leto and load the new save.
Now the first Leto comes in. Take it out and simply copy your old stats over the new ones. Save and bingo - you have a two class character with all the skills of a three class character.
You might want to remove some of your experience points though, otherwise you'll level up instantly.
The only problem is that you might not be able to use some class specific items like The Unmaking, even if you were a Rouge before. Npt that you can't switch out the classes later.
Hope this helped.