How is the tank controlled? Does up on the d-pad move it vertically up the screen, or does it make the tank move the way it's facing? Do right + left move the tank to the sides of the screen, or do they rotate it clockwise and anti-clockwise?
I'd be tempted to include another method for controlling the tank + turret, without needing the d-pad, but I'm not sure whether it would suit your game/engine.
I'd suggest that both the turret and the tank use the stylus. So for example, the turret shoots where the stylus is held, and the tank moves towards where the stylus is held. The tank could have a slow turning speed, so it doesn't move there immediately. the maths can be a bit unfriendly, but it's possible.
(Hope that all made sense, it can be tricky explaining in text what's in my head

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