I can't remember my 1st playthrough but I just broke the 3mil XP with all 6 party members last night so they all started getting some HLA points. I noticed that my Fighter/Thief got a HLA point when the Thief level went up as well as when the Fighter one did.
I know that different classes level up at different speeds (on my 1st playthrough, my main character which was a Sorcerer imported from BG1 ended up at lvl 23-24 while Anomen who probably started lower lvl in BG2 ended up with something like lvl 7 Fighter (Dual class) and lvl 30-31 Cleric.
Still, my Monk is higher lvl (he was imported from BG1/SoD so he started higher and I think he levels faster late game) but aside from him, the others were of similar XP ranges. The Wizard Slayer is lvl 20, the other 3 (archer, shaman, Keldorn) are lvl 18. My Fighter/Thief is lvl 14/17.
So unless I'm mistaken, this means that the multi-classes (like F/T) keep on getting an extra HLA point every team each of the class levels up meaning they end up with quite a few more points than others or is this stopped and balanced out in some other way? If it stays that way, it's a significant end game power spike that I had underestimated vs single classes/kits (on top of having more options).