It is still good practice to isolate a "sick" tank, even though you might have dormant ich in the other tank.
To really isolate the tanks, you need to sterilize everything between tanks. For ich & most parasites, you can either soak in a weak bleach solution (and rinsing well & dechlorinate afterwards ... too much work) or soak in a solution of potasium permaganate <what most
lfs use>. A simplier way is to let the buckets, etc dry completely between tanks. Not as effective as pp, but Ich is mostly dead when totally dried for a day or so. Hot water might help, but to truely sterilize, you need boiling water.