I have a M-Box Pro interface for ProTools.
I can combine two inputs on my interface and send them to one output but, I have to run the signal to my computer first.
The M-Box has software on the computer, with it's own mixer that is separate from the ProTools mixer.
I would have to run the two inputs on the M-Box, into the M-Box Software and back to a single M-Box output.
I'm going to have a look at the Focusrite manual.... brb
Looks like your Focusrite has a software mixer like my M-Box.
If your Focusrite can send an input signal its own output section, without turning on your computer, you should be able to do it from one of the output jacks.
The M-Box can't do that without using its mixer on the computer.
If your inputs send a signal to the headphone jack, without needing to turn on your computer, you may be able to use the headphone jack.
Not sure if my M-Box can do that because, I never tried it!
BUT, if yours can, you would need an adapter to convert the stereo headphone signal to mono and you would need to match that signal to the input levels of your Kemper.
(Edit): A regular guitar jack would convert the stereo headphone jack to mono.
Not sure how your would go about converting the headphone out signal to match your input level on the Kemper - it could be a simple matter of slowly turning up the headphone volume but, I don't know if that will screw up your $2000 Kemper!
I'm tapped out.