I set my multi-wiki droplet up after lots of googling, but I used their Ubuntu LAMP droplet template. That's why I can't really help here on that part, or with CentOS at all. How I did it only somewhat resembled the process that's been described, which may be a better practice method than how I hammered it into functioning.
It does have one domain and multiple subdomains each pointing to a different wiki, which has a shared section for some of the files generic and in use for all the wikis on, with symlinks for those shared folders. (I forget if the one I have now because I keep forgetting to take it down has two or three running on it.)
Am still happy to help you set up interwiki at some point in the future if you're doing the core + supplemental wiki thing and you want them to be talking to each other. That's the main reason I set mine up with multiples on one droplet, since I could also set it up to share the various styling, resources, the reference graphics I'm using, and for what was going to be a later iteration, the skin mods I was hammering on, rather than duplicating them all three times and bloating the hell out of things: they were all going to be linked up through interwiki.