Actually, I am currently doing the very dual class you suggest. The character is now a 24/28 fighter/thief and is (unsuprisingly) very powerful.
I'm a bit of a powergamer so I make notes about what proficiencies to get, what weapons and armour to use, where to spend thieving points, what level to dual at etc etc. If you want my notes for this character, just message me and I'll send them onto you.
Actually, if you *want* the characters save game, I'd probably mail that to you. Save you going through SoA yourself
The party can enter the elven city whenever you like to take on Irenicus.
lol