Not quite. I think...
I thought you could only dual class from 3rd level and up... still, that only makes a difference if you're playing BG1
But then it would be
1st level thief to 3rd level thief: everything is normal
3rd level DUAL CLASS
then 1st level fighter, no thief abilities
When you reach 4th level fighter, you regain your thieving abilities.
Like Lord Shield said, you cannot thieve while wearing anything larger than studded leather. You can backstab with the following: shortsword, dagger, longsword, katana....no blunt weapons etc
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Melusine
Your voice is ambrosia