Install and print from two identical printers on iMac?
Dymo label drivers are quite picky and ask that you not connect the devices until the installer asks specifically for the device to be connected. I believe that is to prevent both queues going to the first device in the USB chain, but I don't have two to test with.

I would delete both printer definitions and start with the installer from here:


https://dymo.custhelp.com/app/answers/detail/a_id/101/~/dymo-drivers-and-downloads#lw_mac


The read me has the following caution:


  NOTE: Do not connect the USB cable to your computer or USB hub until
  instructed to do so during the software installation. If you have a
  label printer already installed, disconnect the USB cable from your
  computer before running the installer and reconnect the cable when
  instructed to do so.


So you could try deleting only one queue and then disconnecting the printer that printed from the other queue and try setting up the second printer again.