100 200-pounders (9 tons) all 10m back from the main wheels in an A320? Versus 60 tons in total. CoG without the passengers has to be nearer than about 1.5m to the main wheels for it to tip.
350 200-pounders (32 tons) all 20m back from the main wheels in a A340-600, versus 300 tons in total. CoG without the passengers has to be nearer than about 2m to the wheels for it to tip.
I'd expect the nose wheels to be at least bearing 10% of the overall weight, so
with a 12.6/32m wheelbase, neither an A320/A346 seems likely to tip. I understand rear-engined aircraft can be more teeterey.