I've never tried to do this. I've not seen any instructions anywhere else for doing it. Its a long while since I've done an install so my recollection of the install options might be incorrect. Here are a few different ways I would tackle the problem.
If I had two disks I would the two versions to the separate disks, selecting the FreeBSD boot loader. On startup the boot loader gives the option of booting from either disk, defaulting to the one previously used.
If I had only one disk I would install the two versions to two different '"slices", selecting the FreeBSD boot loader. On startup, the FreeBSD boot loader gives the option of booting from either slice, defaulting to the one previously used.
If I had limited physical access to the system and I had time on my hands I would try either of the above variants but use GRUB as the boot loader. (I seem to recall some recent reports of problems with GRUB as the boot loader but I don't recall the details. I've never used GRUB to boot FreeBSD. I have used GRUB to boot Linux systems.) GRUB would probably be better suited to unattended operation than the FreeBSD boot loader in that you can specify a default kernel in the GRUB configuration file and the default is unchanged by booting a non-default kernel. Thus you could make (for example) version 1.2.3 the default and version 2.0 as an option. Then if you boot version 2.0 and the system reboots it should boot version 1.2.3.
I don't know if there is a way to get the FreeBSD boot loader to forget what was last booted. But this would probably not be an issue if you planned to run version 2.0 only while you are in attendance.