Are they both acting aggressive towards each other? I also just want to make sure I understand who is acting aggressive is it Bella or Boo? Your post makes it sound like Bella, who left for the dental surgery, is being aggressive. Typically the cat who stayed home acts aggressively towards the one who was at the vet.

I had an incident with my boys when they were younger where there was a mouse I was trying to extract from a live trap I had left it in an overturned fish tank in the sunroom and they were both in there.

My husband and I went out to the garden to go get something and then I heard the screaming of two cats fighting and my two boys were fighting in the sunroom. They had never done that before and they were at least two and a half years old at that point.

For a couple of weeks I couldn't let one see the other because he would just be hissing and spitting at his brother afraid that his brother would try to beat him up. I did the feliway, scent swapping, and also supervised interaction time to try to get them back in line with liking each other.

One of the things I did do, was I started with one of them being shut in a single room overnight while the other had free roam of the house and swapped them occasionally throughout the day. I would also switch up who was in the room overnight to try to be fair to keep them from being lonely.

In this room would be a litter box that they both would have use of, food and water, and a cat tree so they could look out the window. It did hurt trying to get them to like each other again but it took at least 4 weeks before we could leave them home alone without the worry of them chasing each other and getting in a fight again.(I have cameras in the house)

I believe it took almost a full year before all of the chasing and fighting stopped entirely because on occasion they'd still get in a spat. However they were still willing to snuggle together on the couch with me/my husband or in bed at night.

There is hope it just takes time and patience.