Split DNS for 2 different dns names

we have an internal url and an external url but want to leverage F5 DNS to dynamically send traffic to our primary site and if those devices are down go to our disaster recovery site. how can this be accomplished? we have LTM F5s as well that have APM and they will be doing the external url, and want to send the internal traffic to the F5 LTMs per site. Please let me know.