Oh no poor Corkscrew. Yes, I think that you should at least call the vets and let them know that he seems to be getting worse. Since he weighs a lot more than Tibby he may not be getting enough medicine for his weight. This happened to me with my RB Pepper. All 3 of my cats came down with URI and Pepper got worse instead of better because he weighed more and wasn't getting enough medicine. Prayers and positive thoughts are on the way for a full recovery for both Tibby and Corkscrew.

BTW didn't Corkscrew have something similar to FIP? If he did then maybe his immune system isn't as strong as a normal cats. I'd also ask the vet about this. Good luck.