Is it valid to install FME Server (version 2023.1 Build 23619 x64) on 2 nodes (2 separate express installs) with both the nodes sharing the same FME Flow System Share and FME Flow Database (postgres sql DB)?

I think that should be fine, but you'd want reverse proxies to be routing to each machine

• proxy1 routes fme1.com to machine1
• proxy2 route fme2.com to machine2

At this point, i'd argue it would be much better to just setup one load balancer and check the status of each machine and if machine1 is down, then the request goes to machine2. Both fme1.com and fme2.com can be pointed there, you could even still set it up that fme1.com goes to machine1 and fme2.com goes to machine2, but if one of the machines goes down, both urls will point to the active machine

Having clients swap between URLs is going to cause headaches, because things will get hard coded, then process will fail, and need to be updated to switch over, etc.