Answer: I think you're seeing transplant shock and why only one is affected, I really don't know. The best thing you can do for your trees is to move them into a shadier spot, at least until they re-establish their roots and begin to show new growth. As a rule Japanese maples prefer morning sun and protection from hot afternoon sunshine - at least while they're young. I'd move the pot into a more protected area for now. The puny one should perk up and the nice looking one will continue to look good.
Best wishes with your Japanese maples!
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