Non-recognition aggression

My 2 cats, Luna and Charcoal (age 4), recently went to the vet for their annual exams and vaccines. We were there a little longer than in the past (this is a new vet since we moved last December). When we got home, I let them out of their carriers and Charcoal started sniffing Luna's rear. This went on for a few minutes and then out of no where, Charcoal hissed and attacked Luna pretty violently. They were adopted from the same foster home 3 1/2 years ago. They got a long famously as if they were real siblings. Now they act like they don't know each other. I bought a Feliway Diffuser. I kept them separated (which is hard as we live in a loft with only one door (the bathroom)). It has been several days and they are tolerating each other as long as there is a few feet between them. They aren't attacking each other, just hissing and chasing. Should I let them resolve their own differences or should I intervene? If I should intervene, any suggestions on what I should do? Thanks.

I go through this whenever one of my cats go to the vet. They come back smelling differently. Take a towel and rub one cat everywhere, then rub that scent all over the second cat.

I usually also use vanilla above their nose, under their chin, and at the base of their tail. Now everyone smells the same!

They are at it again. I had to take Luna to the vet for teeth
cleaning. Brought her home and Charcoal didn't recognize her. I tried
all the scent exchanging tricks, but Charcoal didn't falling for it.
She is hissing, growling, and attacking Luna. We think the vet
powdered Luna with something (Eau de Vet?). She did smell strange to
me too. It subsided after a week last time and I am hoping it will
again. Should I let them work out their differences or intervene and
keep them separate for a while? If I should intervene, what advice do
you have about doing this? Spray bottle? Loud noise? Last time, I
ended up taking Luna with me because I had to leave town on business
and didn't want to leave them alone.

Mary
California