If you're looking to simply keep two DNS providers in sync, you can just set one up as a Secondary DNS. Dyn provides this as a feature and that keeps our DNSMadeEasy and Dyn accounts synchronized without having to think about it.
Secondary DNS functionality is usually implemented with AXFR which doesn't support advanced functionality like Geo DNS and also limits the providers you can choose as only a few offer it.
It also doesn't help with managing the records in the primary provider, requiring either working in a UI or writing using software like OctoDNS to manage it. It's definitely a good option for simpler and stable setups, neither of which is the case for us or many of the people I've talked to. If it fits a situation you have it's worth exploring.
