It could be scent, first from the basement and second from the vet. I think the best thing is to keep them separated for awhile, let them "cool off", then let them back together, but always separate them at the first sign of "trouble" and long before it comes to blows.
Here's how I think about these kinds of things: there is some number that represents their relationship and current attitude towards one another. When the number gets high enough, they get friendly, when it gets low enough, they are hostile, and all other states in between. Everytime there is a positive interaction, like them eating food that they like together, or treats together, or they sleep next to each other, or they play together, or groom each other, or enjoy the warm sun on a window sill close to one another: the number goes up. Everytime something bad happens: they hiss to scare each other, they growl, or box each other on the nose, or stalk each other, or worse, the number goes down. And the amount it goes up or down depends on how good or bad the episode was. So the goal is to "tweak things" so that you build up a ton of positive capital and avoid negatives as much as possible. To me that's sort of the guiding principal of having cats get along. So if they are in a state where everytime they are together, the only thing that is going to happen is negative things... you don't want them to be together. Keep them apart! Then start slow. A delicious dinner with them only seeing each other from across the room. A small positive rather than a negative. Good! Smelling each other from between a closed door while being stoked on the head by their owner and no hissing or growling. Another small positive. Etc. Eventually it adds up, and they are ok together again. The opposite of course is also true. If they just keep having negative interaction after negative interaction, pretty soon they are going to loath one another, then you've got a really big problem on your hands.
Anyway, maybe I'm mental, but that's how I think about it, and have had luck with it.
_________________ The little monsters 3
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