It’s all about the outputs available and my card has more or less the same configuration : from what I see, the UR12 is basically also an USB unit, with a given amount of outputs and a stereo headphones one, the main difference related to outputs being that yours has just two RCA ones.
So, if you are using the Yamaha Steinberg USB ASIO driver (I know its name thanks to this thread), you should have the four outputs available in the Device Port column of either the Audio Connection > Outputs pane or Audio Connection > Control Room one.
You can check also this in the Studio Setup window, Audio interface > Yamaha Steinberg USB ASIO pane : I guess that you should have all the four outputs of your unit listed as I/O Out ones. If two of them are displayed as inactive, it’s because you haven’t created an added stereo bus for your headphones in the Control Room pane of your Audio Connections window. If the case, in the Control Room pane, right click in the Bus column, choose ‘Add Headphone’ and add your ASIO driver and the corresponding available Device Port items fort it.