I too have cottonwood trees, two were planted at the same time and one is doing ok the other seems weak.
they get the same amount of water
the weak tree just doenst leaf up as nice as the other one.
they are just now gettinr their leaves.
any ideas ?

Unless they're mature and very nice specimens, I'd remove them and go with a better selection. They're likely to give you constant problems.

I don't hate cottonwoods like every one else does on this forum. Yes they can be messy, drop limbs, attract japanese beetles, root flare from hell, and cotton in your air unit, but I like them. Especially in a pasture, they are awsome and grow fast. If it is dying, then cut it down into to sections and inject it with native mushroom spores. Good eating for years.

True - with sufficient acreage, many "trash trees" become much more acceptable.