So I was wondering, if I am flying bossk and use the mangler cannon at range 3 and I roll only 1 critical hit, would i be able to resolve the combat in this sequence?

Trigger bossk to change my 1 crit into 2 hits

then trigger mangler cannon to change 1 of the hits into a crit

then since i am at range 3 trigger mercenary copilot to change the other hit into a second crit

This would effectively turn my original roll of 1 crit into 2 crits.

Is this allowed or is there a rule that I am missing somewhere that would prevent this sequence from occuring?

Unfortuantely, you can`t do this. Mangler Cannon and Merc Copilot happens at the modify attack dice step, while Bossk happens at the compare results step, and only if the attack hit.

Note how on the cards they refer to dice results by using symbols (not the terms "hit" or "crit"), while when the cards use the actual word "hit" (as on Bossks pilot card) it has a specific meaning. An attack "hit" when you have more hit/crit results than your opponent has evade results after all normal dice modifications have been done(with the exception of Lt. Blount who always "hit" even if you only roll blanks.)

So if you rolled two hits, you could use mangler and merc to flip both to crits, and then if your opponent got 1 or 0 evade, the attack would be a "hit", and you could use Bossk to change 1 crit to 2 hits

Ok, thanks for the clarification. That helps me understand the combat sequencing a little better.