Oh, no.
I'm so sorry for your loss.
Please check your husbandry to make sure something in their environment is not responsible. Certain toxins and mold can lead to sudden death. Make sure you are cleaning and DRYING their brooder at least once a day, that they have dry bedding and that their water container is being cleaned regularly (again, daily is best). If they are eating food from last year, throw it out and buy fresh, even if it looks and smells fine (I learned this the hard way). If they are eating medicated chick feed, switch to unmedicated. If they are getting treats other than the crumbled baby poultry feed, make sure they also have access to grit.
Of course check that there are no foreign substances, bits of plastic, etc., in their brooder.
If all that checks out, then you'll just have to wait and see. There are contagious diseases that could cause this, but it's hard to do anything about a disease that kills that quickly.
It's also possible that it's just a fluke. Young birds sometimes just die, for no apparent reason. But when you have two die within a short period, it's always best to check all the relevant factors. If you still don't have any clues after that, keep posting and see if you can't find a vet who can help.
Good luck, and I'm sorry!
P.S. Edited to add: Also, if you think everything else is fine, consider buying a new bag of feed (but don't toss the old one yet), and switching to it. Sometimes a bag of feed, even brand new, can be misformulated or contain toxins or mold. Switching to a new bag can eliminate the problem. If you continue to have problems after switching, then you know the old bag is probably fine too and can go back to using it; if you never have a problem again, I'd suggest tossing the old one, even if you're not sure it caused the problem.
