That is a bit unusual for the joey to be away from one of the parents long enough to get cold at 2 days OOP. I would definitely keep a very close eye on the joey.
If the joey is rejected, or simply needs supplemented and kept with the parents (best option if possible), here is a bit of info that may be helpful:
Q: What do I feed a newly rejected joey?
A: You can mix up a formula made with Wombaroo Milk Replacer, Brisky's Booster Milk, or puppy Esbilac formula. If you use Wombaroo or Brisky's, follow the instructions on the box/container. Mix the Esbilac Powder 3:1; 3 parts water to 1 part Esbilac powder. If the consistency is too thick, add more water. It should flow through a 1cc needle-less syringe. IF the joey is dehydrated, then substitute the water for unflavored pedialyte. Once the joey is back to normal, you can switch to back to using water in the formula mix. You can also get 50% Dextrose Solution from your vet to add into the formula. It is added for flavor and to maintain sugar levels.
1 teaspoon Esbilac Puppy formula powder
1 Tablespoon Water or Pedialyte (unflavored)
.5 cc Dextrose (must be obtained from your vet)
(Keep refrigerated, discard unused formula after 24 hours.)
You can find more info HERE
. Good luck and please keep us posted!