I came across this interesting bit about multiple Mastodon-instances on a single server, howbeit, it's from 2018 and there have been a lot of changes since then.

The author wanted to run two instances off the same base code, different databases, re-using as much existing infrastructure as it could, essentially, and appears to have done so.

I don't know why anyone, other than wanting to migrate their instance to a new one, would do this, but here it is. #MastoAdmin

https://nfgworld.com/multiple-mastodon-instances-on-a-single-server/

Multiple Mastodon Instances on a Single Server

Back when I still ran a Mastodon server, I had a brief need to run two of them on one server. With Gargron's help, I worked out how.

^NFGworld