From a technical perspective, if you're doing fancy DNS things like geo targetting, round robin though more A records than you'll return to a query, or healtchecks to fail out ips from your rotations, using multiple providers means they're likely to be out of sync, especially if the provider capabilities don't match. That may not be terrible, because some resolvers are going to cache DNS answers for way longer than the TTL and you have to deal with that anyway. You'll also have to think about what to do when an update applied successfully to one provider, but the second provider failed to apply the update.

From an organizational perspective, most enterprise DNS costs a bunch of money, with volume discounts, so paying for two services, each at half the volume, is going to be significantly more expensive than just one. And you have to deal with two enterprise sales teams bugging you to try their other products, asking for testimonials, etc, bleh.

Also, the enterprise DNS I shopped with all claimed they ran multiple distinct clusters, so they should be covered for software risks that come from shipping the same broken software to all servers and having them all fall over at the same time.