When doubling cake mix recipes, do you also double time you mix it or would it be the same as if it was just one mix?

Mix time would be the same. Just be sure to have enough room in bowl and that the beater isn't drowning in the batter so that you know all the ingredients are getting fully and completely incorporated.

I've doubled recipes many times and never had a problem using the usual two-minutes!

I mix mine about the same amount of time. I do this even when I use 3-4 at a time. It may take a little bit longer, but not much. I just sort of eyeball it to what one mix looks like when mixed. Never had a problem.

Ditto for me too. Make sure that you do not have too deep of a bowl. I ran it to that problem & the mix almost went in my mixer.