- GitHub Pages only allows one custom (apex) domain.
- Technically: only one entry in the CNAME file
Probably the simplest solution to have another (apex) domain, is to have a redirect. This is implemented here. It is a quick solution, but not a particularly good one. There is some debate about redirects and masking.
Better solution (but more work) is to have a copy of the repo with another CNAME file (could create a fork), but not today. We should do this (once time allows).
References