I attached a picture of how I think it should be wired. Obviously there are colors involved but I think that what I drew is parallel. The tender unit would be attached to the boiler unit if it was in series. I think.
Sorry for crossing the wires right off the bat. I hope the idea comes across anyway.
EDIT: I showed the end of the 4 pin connector coming from each unit because I think there's only one 4 pin on the board to connect to. I don't know if I'll be crimping a new 4 pin connector with 8 wires or if I'll have to make connections elsewhere and bring them to the connector.