One bath per frog and the bath water can only be used once.
Don't try and double doses to have two frogs together.... The salt doesn't "know" to divide evenly and you could really hurt your frogs.
Using bathwater over risks infection due to risk of bacteria forming, concentration or dilution of salt due to absorption / evaporation....
Just too many risks.
Epson salt is very cheap and the quantity you use per bath will make the container last a while.
Unfortunately, you wouldn't have had to do as long of a treatment if they had been treated earlier. Now their bloat is quite severe and needs longer treatment.
Can you post updated pictures of all the frogs?