Ah, good question. I think they will eventually sum together, when Fighting skills return (up to a max of 5, of course). However, I may be wrong. Dual-classing is a bit of a gamble on things like that, and I know the game treats things differently than in PnP.
I would imagine that if you opened up the file in DaleKeeper (just to take a look), it would solve the question for you there. What you could do is, if you have Dalekeeper, is dual-class, allocate the point, save in a DIFFERENT slot, open up Dalekeeper, give the character the XP needed to get him up one level higher than his last Fighting class, then re-open the file in IWD to see what occurred. That would answer the question one way or another. Of course, assuming you didn't want to cheat, you would just revert back to the other save and play it out, or reload the save before you dual-classed and allocate the point elsewhere, if needed.
Cheers,
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Patience, my BUTT...I'm going to KILL something!!!