I have seen very few satisfactory answers to how to use Play, or really any multi-out in Studio One. The answer is fairly simple. In Play, go to the Mixer tab and assign your instruments to the outputs that you want. Then in the mix of Studio one, click in the Instruments block (tab, whatever it's called), make sure that you click on Play so that it displays the output ports, and the make sure they're selected. The you'll then see your mixer channels for your EW instruments in the mixer.