Normally, when a Paired Mage turn order card is drawn and neither Mage has taken a turn this round, you choose which one goes the first time, and the second one goes automatically the next time. 

 However, if a drawn turn order card in the pair is returned to the deck before shuffling, then the choice of which Mage goes does not necessarily happen with the first card each round. So after that happens, you'll see the first card choosing a Mage automatically, and the second card asking which Mage should go.

Note you can always hover over a mage to see which one will go next when their card is drawn.