I don't know if there's an easy way to deal with two frogs in the same tank with this problem. The picky frog may have poor eyesight, or illness, or may just be picky. It's hard to say. Luckily, I've noticed that my frogs habits have changed over time, so one is less picky. It may be the same for you. How's the frog's weight? Okay? You might try making sure there are plenty of places to hide in the small tank to make the picky frog more comfortable. Then I'd just hope it's appetite would improve.
I'd try to see if appetite picked up after a while and maybe reintroduce them if there are no signs of illness.
Good luck!