Correct me if I'm wrong, but I'm just thinking outloud.
When one of your fish have Ich, wouldn't removing them from the tank and treating them in the other tank just defeat the purpose of treatment?
I recently had an outbreak of ich on my neons, I could have put them in another tank, but I treated them in the tank they were in. I wanted to ensure that I didn't get another outbreak in a couple weeks, and to me, that meant treating the water they were in. To kill any future parasites that may have fallen in the gravel, so that the other fish don't end up getting it.