Using identical Central and Gun Markers is common. It's usually Aimpoint or O-shaped. I've been using double Aimpoint for several years without giving it much thought. I average around 4k DPG on some Tier X tanks, but I can't say my aim has ever been a strong suit.
I think I've just had an epiphany. I am finding it significantly easier to aim for moving small targets (such as cupolas) when my Central Marker is different from my Gun Marker.
I think my eyes sometimes play tricks on me when both markers are dots, and switch focus between them too much, slightly throwing my aim off. It feels like setting my Central Marker to A-Shaped makes the brain process everything a lot smoother.
If you've been a double Aimpoint user for ages, it might benefit you to mix things up a bit.
Anyway, I was wondering if anyone had a similar experience.