Thank you for the reply and I do understand that. However, would that make it only have sound come out of 1 speaker? Shouldn't I hear the summed sound coming out of both?? I'm going 1/4" left out (or right out, doesn't matter) to input 1 on a Focusrite Scarlett 18i6 interface that feeds 2 M-Audio monitors. I would think I would hear sound out of both but am only hearing sound out of the left monitor.
Any other time I've run a guitar modeller/fx/etc...it's summed like you said but I'd get sound out of both monitors using only the 1 input as stated above.