Problem with backstab

Hello friends! I'm having a little issue, 'd you like to help me please? Im soloing bg2 + tob with a dual figther/thief char. In the first box the STAFF OF MAGI, in the second box a long sword. Ok, so, im invisibility at the back of an enemy and change the weapon to the long sword i lost the invisibility before i can backstab it. If i don't remember when you change the weapon the invisibility last one round.. right? any suggest? THANKS!!!!
hugs from argentina