While it is possible to have multiple OpenBSD systems in separate MBR partitions, it requires third party boot managers and there are configuration limitations. I've done it, but there are easier ways to manage multiple OpenBSD systems on a single computer. The easiest method is to use multiple drives, as
ocicat recommends.
The second easiest method is to install a second OpenBSD sysem into the same MBR partition, just into a new disklabel partition created from unallocated storage You can select the kernel and root partition at boot time. And the easiest way to install a second system onto the same drive is to install first onto a second drive, such as a handy USB stick, then replicate its contents into the new disklabel partition back on the first drive.
Yes, it is possible for someone to do install a second copy of OpenBSD without a second drive, but it is far more difficult. And error prone. There are two methods: manually unpacking the filesets and deploying /etc infrastructure manually, or the far more risky path of manipulating disklabels and DUIDs from the RAMDISK kernel in order to use the install script with the new partition temporarily defined as "a" partition during install and without overlaying your existing system, then manipulating disklabels and DUIDs again once the installation is complete, and then finally editing the new fstab(5) to permit addressing for the restored DUID.
If the above paragraph seems ridiculously complex, risky, tedious, and prone to failure, then ask someone who's done it multiple times. Me. It is even more complex, risky, and tedious than described. Doable? Yes. Did I backup first? Every time. Did I ever need to restore? ... ... Yes. Once I had to restore /etc files that I fumbled into destruction. I've also had to reinstall boot blocks from installation media. More than once. More than twice.
Now, if you really want to have OpenBSD systems in separate MBR partitions, then....
...Let us suppose for the sake of argument: the world is flat, and you wished to sail off of its edge. If so, the map that you can follow to the edge of the world and beyond can be obtained from the sysutils/grub port. It is not a detailed map. Nor does it say, "There Be Dragons."
The contents of the OpenBSD specific README for that port will be installed into
/usr/local/share/doc/pkg-readmes if you install the
grub package from your nearest mirror. But you can also read it online, without installation, directly from the web portal to the OpenBSD CVS repository
here. The instructions are not a step-by-step how-to, they are general guidance for the provisioning limitations of multibooting a configuration that has already been configured with multiple OpenBSD systems in separate MBR partitions.
Alternatively if you have modern hardware, it may be capable of running a hipervisor that can host OpenBSD as a guest virtual machine. But that's out of scope for this discussion thread.
--
Here's the TL;DR - If you want to have multiple OpenBSD systems on a single computer, both
ocicat and I recommend you use multiple drives.