I recently dualed a character. Now I knew ahead of time that the original class would be unusable until I caught up to the level I dualed at.
Then I gained a level. Am I understanding correctly that there will also be no hitpoint gain until I catch the original level? I got NADA.

thanks.

What class did you dual to, and what from? I have never heard of not gaining HP at all, I find it very odd. Maybe it was just a bad roll with too many penalties? I was not even aware that it could be less than 1. If you have a save when they leveled up, try repeating the level up and see if they gain hitpoints then. Or, just wait for SixOfSpades or Dundee or someone who knows this to come along.
-Brainless One

If you have, say 5 levels in the first class, you will not gain hit points with the second class until you reach 6th level.

-Echon

If you dualed from a fighter to a mage, you will not gain hit points until you reach the level at which the mage would have more hitpoints than the fighter.

Echon is correct--it's based on levels. You can never have more than 10 Hit Dice. If you Dualed your Fighter into a Thief at Level 6, for example, you would have 6 Fighter hitdice, then no hitpoint gain at all for Thief levels 1-6, and then 4 Thief hitdice at Thief levels 7-10.