Broxi I'm curious why you would want/need that ?
Multiple hosts with the same content is usually regarded as very bad for SEO. It can also bring many issues around cookies/login state and just about confusing users.
As for http/https mix, this shouldn't even be a question in my opinion. Your forums should always be served over HTTPS.
You should be able to just setup (permanent) redirects from those alternate hosts to your canonical forum ?