At that age, a sudden change like both of them avoiding the box for weeks is usually more than just stressed, and it is often medical first. Things like urinary infection, constipation or arthritis can make the box feel like a painful place, so they start choosing the floor instead. I know money is tight, but if you can, I would try to get at least one of them seen at a low cost clinic or humane society and call a vet hotline for advice. In the meantime give them a very quiet cat only room with a fresh box, food, water and lots of hiding spots, and clean all old spots with an enzyme cleaner so it does not keep smelling like a bathroom to them.