Mine were fostered together for a few days right before I brought them home and reportedly got along fine. Got them home (together -- same time) and they acted like total strangers who hated each other on sight for days. Got them settled down & settled in & about 5 weeks later had to take one and just one to the vet -- we were gone maybe 2 hours, since an emergency came in just as we arrived -- and then we came home to no fewer than 3 days of non-stop hissing & growling from she who had stayed home. The hostility level was so high I actually called the vet back and asked for advice, as I had previously been just a single-cat household & wasn't adequately familiar with the multi-cat dynamics associated with a *short* trip to the vet. And that's exactly what the vet's office staff said -- she smells funny, should be fine soon. 3 days seemed a little long to me, but it did pass.
If tensions remain, you may want to try a Comfort Zone by Feliway diffuser, which sprays feline facial pheromones (the ones that make them feel happy & at home) into the air every little while. I can't smell it, but they can. Mine developed other issues after a serious fight and the issues lasted for about 6 weeks before I finally broke down and tried Comfort Zone that had been recommended to me and I was *stunned* by the difference it has made. The things aren't cheap, but they are cheapest, from what I've seen, on Amazon. And while I was a huge sceptic, I do think they work.