If you are connecting via that hostname, then on WAN link failover DDNS will update that entry with the IP address of the second WAN port. As long as the client is connecting to that, it should work.
The other consideration is if the MX has the public IP directly on its second WAN port, or if it is sitting behind something doing NAT. If it is sitting behind something doing NAT, and that device will need to port forward udp/500 and udp/4500 to the MX.