It worked pretty well. The sergeant didn't have a comrade to direct but was busy directing everyone else plus manning the pintle heavy stubber, so it all worked out fine.
Cool. How many tank combats did you play out? And did your players enjoy their roles within tank combat, limitations and all?
Quite a few. We played the Against the Savages mission, which worked out well - disembarking from the transport, trekking off to the refinery (they picked up the stormtroopers and had them play tank-riders, so they had someone to man the wall defences when they got there), taking the refinery and defending it.
We played an adapted form of the 'behind enemy lines' mission from the GM's kit, too - mostly because I wanted to do The Beast in 40k.
It seemed to go fairly well. The Sponson gunners are arguably more important, because the attack forces always include both battlewagons and buggies and infantry, which the heavy gunner can't effectively engage. Taking a vanquisher specifically stops him having the "anti-everything gun" and makes those heavy bolters vital for keeping you alive.
Also, tended to make them fight in the open every so often. Orky antitank mines are lethally powerful and blatantly obvious - but can be crept through by infantry - so sometimes they need to advance on foot to reach an objective.
Plus we had one big massed battle. About a dozen tanks plus one super-heavy bulling its way through the middle of the fight.