How to instruct systemd to use a specific unionfs for /home

Context: Say I have two workplaces. Currently, I carry a laptop along, and have external HDs staying in these workplaces to hold backups. Now, I'm given the opportunity to have a desktop in both workplaces, so I'd rather carry along an external HD (USB SSD) holding my files, and use the internal HDs of the desktops to hold backups. The desktops don't have the same hardware, so I'd keep config files specific, thus not store them on the external HD. The external HD would only hold ~/Documents ~/Images etc. The plan is to have /home mounted this way: unionfs /HomeLocal=RW:/mnt/external_drive/HomeRoaming=RW /home /HomeLocal will hold dotfiles and dotdirs. How should I instruct systemd to mount /home this way before users may log in? This is Linux Mint.