assuming, since this is a bass, you're not using a switch, which means your likely running the 2 pickups together in parallel, and the way parallel works, the current is going to pick the path of least resistance. so when you have one pickup all the way up and the other not all the way up, its going to pick the all the way up for the most part. it will not be a very linear change, its going to be very noticeable.