I would imaging I can repeat the command and it'll just add it to the primary? Does this make one primary and the other secondary or would it load balance between the two?
From what I understand the Adtran 5000 series does not support helper addresses as that's more of a Cisco command. It does support multiple ip dhcp relay destinations though.
IP helper-addresses aren't prioritized in terms of primary and secondary. Yes, you can have more than one on an interface. The ip helper-address command takes specified UDP broadcast packets and sends them as unicast to the helper address(es).
For your scenario, you'll want to deploy a mechanism to ensure that your redundant DHCP servers keep their lease databases in sync on the back end, but multiple helper addresses will indeed work just fine on Adtran devices.