Beating my head trying to figure this out: how to I make an mdraid1 to install on in the CentOS7 installer?

I knew how to do it in CentOS 6, but I just cannot figure out how to setup mdadm to install / on to. Thanks for any helps/links, my google-fu has failed.

edit: Figured it out, however I can't seem to put a biosboot partition on both disks.