Is it "mean" to feed one cat one food but not the other? Allergy issues

I have two cats. One has rather severe food allergies but does well on rabbit-based foods, so that is what I have been feeding both. Chicken and fish seem to cause the allergic one the most issues, so that rules out most cat foods. The other cat seems OK with most anything.

So of course the rabbit-based foods have suddenly become hard to find in my area, and no one knows when or if that will change. Now I've got to find a replacement for the allergic cat and it's just as hard as last time I went through this.

I tried Natural Balance Venison and Green Peas last night - he certainly liked it but I'll have to watch for an allergic reaction. The other cat did not care for it at all. I think the green peas put him off. But I can't put out something else for him without the other cat getting into it so it has to be non-allergic for him, so I'm back to square one.

The other cat doesn't need wet food - he only eats a bit anyway and leaves the rest for the allergic one. The dry rabbit food is still available for now so he can eat that. The main reason for the wet food is to help the allergic one's constipation, which the other one definitely has no issues with (he's younger and healthier).

But I have nothing to give the other one when it is wet food time and it seems mean. What to do?