A flying Colossal figure gains the Carry ability twice. Once from the Flight symbol and the other for Giant size. If you outwit one does it also outwit the other?

And does being colossal on elevated terrain doesn't give you any advantage on line of sight?

If you have two sources of a power you would need to counter it twice.

Only to seeing higher elevations or to see over other characters, but for lower elevations you would still need to be on the edge and it would not help you see over anything a regular character on elevated to a lower elevation.

I doubt this but if you could use the Carry ability twice could you carry two figures?

Wishful thinking.....

Unfortunately not, even though you have it twice each instance only allows you the ability to only carry one character, it would have to be a special rule like Darkseid's.