ZS doesn’t update 2 dyndns, this is a fact. A workaround I had implemented for this was to use 2 systems in my LAN to update the 2 dyndns accounts and with netbalancer rules I forced them use each one a different pppX interface.
Now normally all your traffic to the internet will be using the pppX interfaces, since it has a default gateway acquired from your provider. If you want to use the BOND for that you will have to add it in the Netbalancer as a gateway (the remote IP address of the BOND).
Yes, NAT should be enabled to pppX.