From the manual:
"The Eris active speaker provides a choice of two pairs of inputs on the rear-panel: left and right balanced ¼” TRS and left and right unbalanced RCA. These inputs accept a line-level signal from an audio source and feed that signal to each monitor’s power amplifier. These inputs are provided to enable flexible connectivity, not for connecting multiple sources to your speakers simultaneously."
I would guess if you did try to use both inputs at once it would overload the input, resulting in distortion. I don't see why you couldn't plug in to both inputs and just be diligent about not playing both sources at the same time. Just a guess though as I don't have the E3.5s.