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If your DNS is pretty much standard DNS (basic zone files, or something equivalent), it's pretty simple to use multiple providers using AXFR to transfer from wherever your source of truth is.

If you do fancy DNS, you'll need to manage configuring them all to be 'the same' yourself. Maybe there's some tools out there, but it becomes write a config file you understand, and a script to reconfigure the provider to match, etc.

The fancier your DNS is, the harder it is to synchronize among multiple services. Things like monitoring and removing dead hosts is probably not awful, but if you need to synchronize geographic targeting, that's going to be complex (and unlikely to be perfectly in sync; different DNS servers are likely to categorize IPs differently, and you'll just have to live with it). Many moons ago, I used Akamai load feedback to divide usage between different locations, I don't think that would work that well if using two (or more) services.