I use the cached al-azif payload server, then I just run hen. There isn't anything special you have to run. One hard drive formatted through the PS4 as external storage, the other exFAT plugged into usb0. Packages on internal storage can be transferred to the exFAT hdd in the usb0 spot the same as you would when only using one hard drive. I've never tried on Mira+hen, but I would assume it would function the same.
To install pkgs to the hdd formatted to external storage, you just have to set the external drive as
default install location through The settings menu (settings>storage>options).