On Shiptheory we require the address line 2 to be filled out with shipments being sent via the DX courier, this is because the integration between Shiptheory and DX was built via the API that the DX system and the Shiptheory system uses to communicate with each other. This results in DX shipments requiring the second address line as the integration was built to DX's specifications. 

To send out shipments via DX services without the second address line, you will have to book shipments via DX directly as then the second address line is optional.

If you need any additional help you can get in touch with Shiptheory support and we will be happy to help.