After you dual class a character, do they continue to gain HPs when they progress levels in their new 'field'? I ask because I dualled Imoen into a Conjurer at level 2 (to plan to make Imoen the sole mage of the group) and when she moved up from level 1 to level 2 conjurer, she did not gain any HPs and I reloaded it quite a few times.
She won't gain new HPs until she has reached her old level. Well, actually if you dual her at lvl 6 she will gain more HPs when she levels 7 as mage. Same goes for skills.
The problem is that the people with the most ridiculous ideas are always the people who are most certain of them.