You need to be able to install a certificate onto multiple servers, or else they'd be useless in a webfarm scenario. (Or does the cert actually get installed on the load balancer in that case?)
Here's what I'd like to do, can someone tell me if this is possible?
We've got a production server with a cert installed on it that we bought from GoDaddy. We've got multiple domain names - let's say D1 and D2. The certificate covers both domain names, and we currently have D2 forwarding to D1. D1 is hosted on a Windows 2019 box running IIS.
We currently have some free hosting with our mail host (different company), that uses cPanel/WHM Apache server.
In case the production server explodes, I was thinking that I would update our DNS entries so that D2 points to our free hosting to serve a "down for maintenance" page, and have D1 forwarded to D2 while we're working on the production server.
However, I'd need to have the same certificate installed on that 2nd server so that visitors don't see security errors.
You would need a wildcard SSL at best to do what you are suggesting. I would be more inclined to have the second site(s) have their own SSL. Mixing server types (Windows/Apache) with the same certificate is a bit of a different setup for sure.
There are those with more SSL experience than I have. Hopefully, they will chime in as well.