Point of information - the IL-2 initially started out as a single seater, and while some (number unknown?) were converted to two-seaters in the field, the IL-2M was the standard production two seater - with gunner.
Anecdotal sidebar: ... most gunners supposedly were drawn from convicts, promised their freedom if they survived x number of missions.... but if they were close to completing the number, they woud be transferred. ... apparently IL-2M gunner mortality was high, and IL-2 losses happened when the enemy fighters realized the gunner was dead/incapacitated, because the gun would flop down... a spring mechanism was devised to hold the gun up...
Dedicated dual-control versions of the "Sturmovik" were designated U-Il-2 or Il-2U ...
Of course, totally anecedotal, and I can't even remember where I read the stuff, but...
Hagar, LOADS of early IL-2's were converted to two seaters in the war to incorporate a tail gunner.
result: most dangerous job in the russian airforce, because the armor wasn't that good on the rear side as it was on the front side