Wow.. this sounds like a very stressful situation! As for the smell thing, I have read that if you spray a bit of perfume or something like that on both cats, they will smell like each other and therefore associate with each other better. That might be one option.

But like many have said already, it seems strange that this is a "sudden" thing. I think going to the vet is a great idea, especially, like you said, because cats don't let it be known that they are sick until they are REALLY sick.

Also, senior cats do go a bit "loopy." They do tend to forget things. For example, my parent's 17 year old seems to forget where he is, and goes into the kitchen and howls until someone calls his name and kind of "reminds" him he is where he is supposed to be. It could be that Tubby is just a bit of a grumpy old man

Another thing you might try (depending on how you feel about this) is an anti-depressant used for inter-cat agression. I had Bassett on an AD because she was peeing where she shouldn't, but the vet also said that it was used to stop aggression between cats. It's called Apo-Amitriptyline. You might ask your vet about it, depending on whether everyone is healthy and stuff.

Regardless, please let us all know how things work out. We are all hoping and praying for a solution here!