D/C:ed Shar-Teel as the party thief viable ?

Hi,

I'm planning to do a playthrough with an evil party with the following setup:

Half-Orc Fighter/Cleric
Dorn
Edwin
Shar-Teel

Optional additions: Kagain, Viconia.

Now to the question at hand: If I rush to grab Shar-Teel, when would it be optimal to dual-class her, seeing as she'll be the only thief I'm going to use. I considered 6 for another pip and even 7 for another ½ attack, but I fear that will keep the party w/o a thief for too long.

Thing is, I want her to get some proficiency in longbows, I don't want her in the frontlines with a CON score of 9, so my plan is to set her up as an archer with the option to do some serious backstabbing, utilizing that nice STR score.

How would you go about dualclassing Shar-Teel ? Or would you rather not bother and pick Montaron instead ?

//C