Welcome to the forum! I think the Scarlett Solo instrument input is mono only, so you won't get stereo either way. You'll need a soundcard with two inputs for that. I don't know the Kemper, but I would think most sounds are mono with probable exceptions for some of the effects on top maybe? If however, there are stereo profiles on the Kemper, I'm sure you can set the output section to mono.
Another thing: Are you experiencing troubles with your sound by using just one output? Is the sound much different to using headphones out of the headphones output for example?