If they are on different hard drives, you might be able to get FreeBSD’s boot manager to boot from the second drive - I dont think that should be a problem.
Alternatively maybe look at VMWare Workstation (it’s in the FreeBSD ports too). Then you can even run both at the same time…
If you install Router Os first, then modify it’s partition with partition magic , Router Os can boot with other system , such as windows .
I aready get it now.