Will you carry the party over to BG2, and ToB?
Thank you very much for helping! I will carry to BG2, yes
I think I will only dual class my main at level 7 or 9 and keep Imoen as a thief. In BG2 I will have her as a Thief/Mage anyway.
Well, seems NPCs without mod do not carry over their stats, so change Imoen as soon as you have a comfortable thieving ability. Usualy lockpick or traps.
For the main character, if you carry all the way to ToB, and you hesitate between 7 or 9, it's not even the stats you have to consider, but the amount of exp you can mine in BG1.
You realy want to change class AFTER beating Sharevok in case you don't have the exp-cap removed.
Obviously I have no idea which expansions you have for BG1, but assuming you have (or can have in effect) The Black Pits, I'd change at clvl 7.
Without Black Pits (and no exp-limit removed) I have to say with party the HP is not that much of a deal, and you get 3 companion at the start of BG2, so I'd only go for clvl 9 if you are minmaxing your character.
On the other hand if you have the exp cap rmeoved by any means, there are plenty of more exp in BG1 (easily around a million for a solo character), so THEN I'd go for clvl9.