I just came across a commit on GitHub. How does one go about having multiple authors on the same commit like that?
That's not really two authors; that's an author and a committer. The two fields have different meanings. The author is the one who created the content, and the committer is the one who committed it. When you do a normal commit, you are both. (And both come with an associated email and timestamp.)
But they can become different in a few key ways:
git format-patch / git am - this pair lets you turn commits into patches, generally submitted by email, then have someone else apply them. You remain the author; the person who applies them is the committer. This is pretty definitely what happened on github there.
git commit --amend, git rebase, git filter-branch - These are all basically variants on history rewriting, ranging from single commit to some history of a branch to the entire history. They can potentially modify the committer information. In particular, they always rewrite the committer timestamp. The original author remains in place (in default modes of operation), and if the author is also the one doing the rewriting, their name and email stay, but the timestamp is naturally different.