It's bad.. There is a genetic component to the disease that is, the viral genes insert into the chromosome of infected chickens and the virus will pop out when the conditions are right. Similar to Marek's in that it is a virus that causes tumors - It often targets the liver which leads to many...
I would try to get a necropsy on the dead bird. If you have a local state vet, it would likely be free. So many bird ailments have similar symptoms and/or cause other issues with same symptoms. With 2 birds getting I'll on the same timeline it's suspect. Lymphoid Leukosis potentially, but...
It's not uncommon for adult birds to develop coccidiosis; especially in the spring as the ground warms and things that live within it come to life. It won't hurt to treat with Corid and there is no egg withdrawal while doing so.
I would syringe her some water. The white watery poop is often dehydration. Maybe add a soluble vitamin and/or electrolytes to the h2o. Maybe worth testing for coccidiosis if she doesn't improve after hydrating.
What does their poop look like? If you rub/pinch their feet do they move them? What does her crop look and feel like? Any abnormal odors coming from beak or vent?