I assigned 2 disks to TimeMachine and when 1 goes missing, TM complains, but it should seamlessly continue to use the 1 that is present?
I have Time Capsule and Time Machine setup on my MacBook Pro.
The Time Capsule is always available at home. The Time Machine is only available sometimes.
macOS never complains about the Time Machine drive being unavailable, unless it's been like 7 days (which almost never happens).