On VSX, everything is handled by VS0. All virtual systems will be the same as VS0. When patching and/or upgrading, you only need to do it on VS0, which in this case will be what you refer to as "FW1" and "FW2".
When moving between major versions, aka going from R81.10 to R81.20, you will need to upgrade VS0 but also run vsx_util upgrade using SSH on the management server to change the VSX configuration on the management server from R81.10 to R81.20.
This is not applicable in this scenario where you are only talking about moving between minor versions / JHF.
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