This may not be relevant, but in the past I have had difficulty booting more than one FreeBSD-based OS from the same drive using the FreeBSD bootloader. The reason is that the FreeBSD boot block code (boot2.c) doesn't just look into its own partition for the boot loader and config. Instead, it scans all partitions for them, and so will always load the first one it finds.
There is a way around this, by interrupting the boot process and entering a string with the disk address of the boot loader for the partition you want. I used to do this, but it's ugly.
This info may be out of date. I haven't tried putting 2 FreeBSD based OSes onto 1 drive for awhile.