It's a lot less hassle if you always save your installers that you downloaded in order to install the software in the first place. If you can't run the installer for 12 without first having an installed copy of 11 due to it being an upgrade installer, you can install both in turn. But you may be able to just install 12 without 11. I'd at least try it.
If you can't find your installers, contact FileMaker tech support and as long as you registered your software, they should be able to verify that you had properly licensed copies of the software and they can send you a new installer. I'd expect that it would not be an upgrade installer either.