Brian is right. You are summing the two sources, and will end up with a much louder mix that either of the two sources separately. As a simple example, I can see this on the output meters of an R-4 Pro, when I am recording four channels. Although none of the channels are clipping, the summed channels shows clipping on the output meters. As Brian indicated, you need to adjust the relative gain of each source, so that the result will not clip. Best to copy a piece out of one of the loudest segments, paste it to a new file, and experiment with it there, until you find the levels you like, and ensure that it won't overload.