Hello! I am conducting a research study where I need to send sounds from 2 different sources to 2 sets of headphones, and I need both sources to play over both the left and right ear. The sounds are being sent from a desktop computer (through an external soundcard) and a laptop. Then, they are being sent to two separate sets of headphones. I believe that I need a mixer, but I am unsure of which one. I found this one (https/smile.amazon.com/Nady-MM-141-4-Channel-unbalanced-Mixer/dp/B0009X9H9I/ref=sr_1_4?s=musical-instruments&ie=UTF8&qid=1523051741&sr=1-4&keywords=audio+mixer&refinements=p_36%3A1253545011) and this one (https/www.amazon.com/Behringer-MX400-BEHRINGER-MICROMIX/dp/B000KGYAYQ/ref=sr_1_1?ie=UTF8&qid=1523052918&sr=8-1&keywords=audio+mixer+behringer+mx400) but I am unsure whether the sounds from the mixer output would only be sent to the left or right channel. My thought was to attach a splitter to the output of the mixer and then attach both sets of headphones to the splitter, but if the sounds are only sent to one channel, only one person would hear anything. Does anyone have any suggestions for a mixer (or other set-up) that would accomplish my goal? Thank you in advance!