Ohhhhhh man it has been a very very very very long time since I have played Baldur's Gate. Mainly because I didn't think I could run it with Windows XP, and that has been my operating system since 2003. But I tried it out and BG:I works wonderfully. And it is so great to be back and playing it.

However I have a question which I really never did solve. I always hated leveling multi-class characters because I don't know which one of their class hit-die I use. So Jaheira gained her first level (druid and fighter) and I don't know which one to use? Do I level with the 1d8 +2 (constitution) or 1d10 +3 (real constitution modifier)

Normally MC character level up one level at a time and get 1/2 (rounded DOWN) of the hp that a single class character would get from that level up.

Jaheira however, hits her fighter and druid level ups at the same time. So she gets
(1-10)/2 + 1.5 rounded down from the fighter level
and
(1-8)/2 +1 rounded down from the druid level

While you might think that this would give you a total of (1-9) + 2.5 rounded down, it's actually a bit worse than that because of the rounding. The worst level up you can get is 3 hp and the best is 11 hp. The average is 7 hp.

Just curious... So because of rounding down, a multiclass character with no constitution bonus can be unlucky to get no HP during any level up ever? Or is 1 the minimum always?

No one is the minimum.

If you have constitution 18 or 16 you don't get screwed nearly as much as if you have constitution 17. So Jaheira really is the worst case because she levels on both sides at once and has an odd constitution.