When installing a second hot water tank (the storage type) for redundency purposes, (1) should they be connected in series or in parallel? and (2) what gate valves and/or check valves should and shouldn't be installed on the water inlets and outlets?

You may benefit from reading this post that came today. Both Tom and Milo are experienced plumber but have slightly different views on te subject. https://www.askmehelpdesk.com/plumbing/hooking-up-2-hot-water-tanks-39662.html My own preference would be to install in parallel with gate valves on all inlets and outlets so you can isolate the heaters individually if one fails, but I'm not a plumber.