Can only speak for my Ambrosia variety but generally the top ear will be ready to pick about 2 days before the lower ear. I find that there's a 3 day window to pick ears at their very peak of perfection before they get a bit tough and starchy....for me.

A lot depends on variety. Stalks that produce 2 ears can have a decent second ear ready at the same time or more likely a few days to a week later.

Same goes with harvest window. The yellow variety Incredible will usually have kernels go sour before they get hard so I've harvested from that variety for up to three weeks.