This blog by user Eric McCorckle documents his experience in running FreeBSD and Gentoo side by side. He has set up a ZFS volume on his Librem 15 notebook to run both of the open source operating systems. Follow the link below for his full story.
My Librem 15 arrived a while back. I normally prefer FreeBSD for just about everything, but I need access to a Linux OS running on the Librem platform in order to help me port over some remaining device drivers (namely the BYD mouse and screen brightness).
In order to facilitate this, I’ve installed a setup I developed a while back: that of a dual-booted FreeBSD and Gentoo Linux system living on the same ZFS volume. This article details how to achieve this setup.
Note that this article is based on the EFI bootloader. If you insist on legacy BIOS boots, you’ll need to adapt the procedure.