Your actually keeping them correctly. I would try butterworms,silk worms if you can get them as they are good to fatten them up.

I dont dust crickets with vitamin powder as my fire skink doesnt seem to like that but I do gut load the crickets with good food few days prior to feeding them this might help you in long run.

For now your set up is great . If your fire skinks arent an actual pair they will fight and 2 together wont work out .

Yours just came in "beat up" imagine wild caught but your set up is fine . I use a UV basking bulb only because my motto is since have a basking area anyway might as well have bulb with UV.
Mine isnt a big basker but light is on 10/12 hours a day.
I have heard some will eat baby food chicken,fruit mine wont but for you worth a try .

Mist them once a day .

The fruit mine gets is from what I feed the crickets otherwise he wont touch fruit either. Good luck hopefully they will pull through