Thank you for the post @VLim
You technically do not have to install base images 9.1.0, 10.0.0 and 10.1.0. The only requirement is this image should be downloaded and you should instead install latest maintenance release of each major version. In this case the upgrade path is: Download 9.1.0 > Install 9.1.11 > Download 10.0.0 > Install 10.0.7 > Download 10.1.0 > Install 10.1.2.
Regarding your plan to upgrade secondary Firewall to target version 10.1.2, it is possible, but not recommended. Please refer to this KB: https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000ClRrCAK Point No. 4 Firewall Upgrade Procedure (HA).
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