I often see YouTube videos of the OP-1 being fed thru guitar pedals like the Chase Bliss Mood and Blooper, and in those videos they seem to have a stereo output signal, even though those pedals output in mono. What is typically used to convert single-channel audio output from a guitar pedal to a dual-channel output?
You can either use a Y cable that goes mini stereo jack to a pair of large mono jack, and preserve your stereo image.
Or loose one part of the signal and expand later the mono signal left with space effects that turn a mono signal into stereo (reverb, ping-pong delay, phaser etc.)
Best IMO is to preserve the stereo signal by using stereo guitar/synth stompboxes such as Eventide, Strymon, Empress, Meris and such.