The cat may be having an allergic reaction to something - could be food, or pollen, plants, perfume or cleaning products.
If he took on both cats, it may be the rescue would assist with any vet checks/expenses which may prove necessary? It does seem a shame to separate them, if it could be avoided.
I still close my eyes tight when I think of how close we came to handing the Gurlies back, when we had a crisis of confidence and thought we were failing with them.
As Rosella's said about Boris and Bobby, we wouldn't be without our Gurlies now, and the thought of them not being together makes me sad.