[CentOS] How to create a secondary btrfs mirror from the CentOS 7 installer?

I just ran through an EL7 install where I wanted to use a lone SATA disk (sda) formatted with xfs for the system drive, then add a pair of SATA disks (sdb, sdc) as a btrfs mirror, mounted at startup on the new system.

I couldn’t manage to do this.  The installer seems to think that if I want btrfs, I want it for the system drive, too.

Is there no way to do one thing for the system drive, then another for the data mirror, all from within the installer?

I realize I can set up the data mirror separately after installation.  (I did in fact do this.)  I was just hoping to be able to be able to document a simple process that a semi-technical user can use to do the whole thing from the installer GUI.

In the past, we got around this by using hardware RAID cards, since they would present as a different hardware volume (/dev/sdb) to the installer, so you only had to turn off LVM to get it to do the right thing.

Now that btrfs is a supported filesystem in the installer, I was hoping to do away with RAID cards, at least for the systems that only need a pair of disks in RAID-1.  

(We’ll still use RAID cards for the larger RAID-6 boxes.)