You're on the right track with caging. They definitely shouldn't be housed together. Start getting some solid feeders in them and some fresh greens...I don't think anyone here would recommend pellet food or anything that comes premade for beardies. Also, if the poo is really runny you will want to keep them hydrated. The sagging skin could be from them being underweight or possibly dehydrated. Sometimes the healthiest dragons can have sagging skin though. Are they active? Do you have any pictures so we can get a better look at them?
I know a lot of people recommend getting fecal tests done but this seems like it was caused by their diet which can be easily fixed and they'll absolutely love you for it.
If they seem lethargic or don't get better from a proper diet, I would definitely take a fecal sample to your nearest herp vet.
Good luck with your new kiddos!