my assassin is now level 22 in chapter 3 but he's playing solo and you've got a party of 5 - it will take you a looong time to get to level 21 (i think) assassin for x7 backstab let alone level 22 fighter afterwards to re-active the assassin
(also i've got tactics mod and 'toughest monster spawns' installed so i get even more XP)
on the other hand you can carry your guy with the NPCs if you don't mind him being weak for a while and not having a thief
if it was me (in a party of 5) i think i'd probably dual at the level at which your traps (snares) stop doing poison damage and start doing fire damage - the poison is much more useful (i think that's level 14 or 15)
on the
other hand you only get 15 thief skill points per level and my level 22 assassin is still poor at lock picking and is using items to help his stealth and trap setting so anything less will be limiting in thief terms
one last thing
is that my assassin seems pretty powerful at high levels even without dualling and you've got korgan for a tank and cernd as a possible melee character so you could just stay assassin unless you have somthing specific in mind that needs the fighter dual . . .
some thoughts anyway (you caught me at the right time as i'm close to dualling my solo assassin to cleric!)