How would you set up simple redundancy between these 2 “cloud” providers?
Does a DNS fallback seem reasonable (that would update records when one is down) or would it take too long to propragate the changes?
A DNS “Fallback” would be the only way but unless we’re talking about a serious server failure would involve too much time between propagation and administration to be able to swiftly react. There’s not going to be much of a “simple” way of performing this task using free providers.
If this involves your revenue stream then it may be worth investing in a less “free” solution that would be able to provide you the redundancy required.