Hey I'm wondering when to dual class Imoen. I also am wondering what thieving skills are needed and how many points to put in them.

I also have a question about back stab. Would it be good for Imoen to backstab as a dual class theif/mage? How does it even work? Back stab and hid in shadows ect never seem to work for me.

Anyway so what are you opinions on the best way to use Imoen?

Depends on what you want from her as a party member. "As soon as possible" and "never" are both perfectly reasonable answers.

Personally I dual-classed her the moment I recruited a second thief in the party (Coran, at which point she was level 6)

The best way to use her varies; that said, having her backstab is a terrible idea. Her low strength gives her an abysmal chance to hit, and when she does hit, her base damage is so low that the multiplied damage is basically worthless.

I do not know how to link a post so I just copied it. I asked a similar question in one of my threads here. Hickory is very good at giving detailed answers to questions like this.

The question pre-supposes BG1. The sensible answer, then, is never. You will never reach sensible dual class level with any character in BG1. The optimum for a thief is either level 9 or 13 for the purposes of backstab points, but for Imoen (who is just too weak for backstabbing without potions) the optimum is probably 8 or 12 for proficiency points. Since you will never recoup all of those levels as a mage to 'catch up' in BG1, it is pointless. Find Traps and Stealth all the way -- forget lock picking, and use Knock or bash those that she can't open. Not a good idea -- see above. The backstab in 2nd Edition is calculated by the following:

((Base damage + Proficiency bonus + item bonus, bard song etc) x backstabbing multiplyer + strength bonus ) x critical hit + additional weapon damage

Now then, Imoen has very low base damage and NO strength bonus, and her chance for a critical is poor. Backstabbing with a low strength thief like Imoen is pointless.

If you dualclass Imoen don't dualclass her to mage, dualclass her to specialist mage (conjurer if you have someone else for casting identify else illusionist) for the extra spell per level and day.
Level 4 gives her an extra proficiency point she can put into large swords, level 5 gives backstab*3 and level 6 is the highest which still allows reaching level 9 as mage. Each thief level also gives 20 skill points and up to 0-2 extra HP compared to mage.

If you already have mages but no thieves in your party keeping her pure is usually the better option. If you want to dual and a short of points to distribute I'd say 75-80 in find traps should be enough for the disarmable traps and I wouldn't give her more than 50-60% natural stealth if any.

Why stealth her up when she'll be able to cast invisibility anyway? I'd actually put points in Traps and Locks (I find bashing is inelegant). or Pick Pockets, if you want the extra cash flow. Yes you can. as long as you have high enough INT (and Imoen does) and the requisite ability score in the ability other than INT (ie.16 WIS for Necromancer), you're good to go.

Because Imoen works best as a scout, and invisibility is inefficient (once per rest) compared to Hide In Shadows, and a waste of a spell slot -- potions are there for those occasions when invisibility is preferred. No, you cannot. Only your primary class may have a kit. Period.

Yes, you can. I've done it. Vanilla, unmodded game. She can specilaize in all but two of the schools because of her high ability scores in all except WIS.

Gnome Illusionist :p

I owe you an apology. I am currently playing BGT, and totally forgot that the original BG1 engine does allow specialist dual classing, unlike the newer BG2 engine. Mea culpa. :(
Ah, but a Gnome Illusionist is just an illusionist. :p

What if she had an item that boosted her strength to 18? I found an item that raises dex to 18 and if I found one for strength and gave it to her, would she be good at backstabbing then?

Gauntlets of Ogre power do exist in the game, which raise STR to 18/00, but I believe they're restricted to warrior classes only, so she wouldn't be able to wear them anyway.
No worries. :)

Yeh, Coelocanth is correct in that Imoen cannot wear the Gauntlets of Ogre Power. However, she can use Potions of Strength, which raise her STR to 18 temporarily, so you could use those for a boost. She still will not have the same THAC0 as a warrior, and a hit is not guaranteed, with the added problem that her relatively low hit points may put her in extreme danger if it's not a one hit kill. She really shouldn't be used for general backstabbing. One strategy, if needed, is to have her carry a potion of invisibility and wear the Boots of Speed. That way, she has a greater chance to get out of trouble if needed.