One of the simple solutions to have a dual boot of FreeBSD and OpenBSD is to use boot0 (man 8 boot0cfg) boot manager.
boot0cfg -B ada0 (Where ada0 is the first hard disk)
From FreeBSD or OpenBSD or bootable CD/USB (Helpful if one system overwrites the other boot loader). It will overwrite MBR, install the boot0 boot loader which will detect other BSD slices and add them to the boot loader.
It will also detect Linux partition and even show connected disks (like USB) to allow booting from.
Unfortunately, the boot0 boot manager doesn't perform well in booting Linux, although it detects Linux partitions and add them to the boot loader, pressing the function keys to boot Linux sometimes don't work.