You are putting both instruments in parallel with each other into the same input of the amplifier. When ever you or your buddy change settings it affects the other instrument. The pickups are working as if they were in the same instrument. This happened to me too during my early days of playing. One of my first bands had two guitars and a bass through the same amp with one 12″ speaker. Talk about a bad sound. I remember playing in bands where the rule was that whoever had the loudest amp played lead guitar! The worst was performing shows where the amplifiers sat in front of you on the stage floor.