If it is crypto, it would almost certainly already have been transmitted to the other one if they are in the same enclosure or were ever in the same enclosure (for example at the petstore), or if you handled one and then the other without washing your hands. Crypto is highly contagious and can even be transmitted by air, people have had it transmitted to a roomful of geckos.
Crypto is always fatal, but other things besides crypto can make babies vomit, so don't give up hope. Did you get the gecko at Petco? They have had many cases of selling crypto-infected geckos. Not a safe place to buy a gecko.
What is the temp on the warm side? Do you have a thermometer in there? You need one.