Being able to smoothly bring up pre-recorded sounds from a second source allows for sound layering, and transitions between times when there is no laptop output. I'm using a SanDisk mp3 player into a Behringer FCV100 pedal. Running that and my laptop into the mixer works great, but to travel light I wanted to trim that down into a small passive box. What I need is two 3.5mm stereo audio input sources going to one 3.5mm stereo out to the amp. I'd also like another 3.5mm stereo out for recording, which needs to be attenuated to mic level.
My first pass at solving this was to build a passive summing cable, then a stereo Y plug with one branch going to a pad cable for recording. That worked, but it was a bit clunky, and was plagued by occasional cutouts and other issues related to treating the computer output as TRS when it is actually TRRS. I've given up doing it as a cable, even though it fundamentally works. If you want to try it that way see that writeup.
Here's the wiring diagram for the mixer. The summing portion is on the left, and the Y-plug/PAD part is on the right.