Stereo blender effect?

Hi,

I play the violin, and I have a dual pickup that consists of two mics outputting to a stereo jack, having the tip and ring outputting the two elements of the pickup respectively.

I was wondering if there is an effect that takes a stereo ¼" jack (or right & left using an insert cable), and has a mono output which is a blend of both channels, with a knob controlling the blend or dry/wet of both stereo sides.

If there is such a pedal, does it have an official name so I can use to search for various makes out there?

Any recommendations or alternatives?

Have question as well

I don't have a Taylor yet, but I'm working on it. I'm looking for info on the Fishman Prefix Stereo Blender system Taylor puts on their 5XX series and above. Does the mic really improve the sound quality when added to the piezo sound? What does the mic sound like by itself? Is good sounding direct recording possible with this system or not?And what a cheap headphones need to get?

Thanks!