I have the same 2i4 interface and use TS cables without issue. (in a home studio with no environmental issues to interfere with signal)
KPA (and any non-mic signal) should only connect with normal TS cables to focusrite interface. If you use XLR, the focusrite engages the mic pre amps on the signal, which is generally not wanted on any line signals like Kemper that you are recording. See article from focusrite below.
https://support.focusrite.com/…LR-Input-on-my-Interface-
As for stereo, you can monitor in stereo with the monitor switch on front of 2i4 set for stereo and the interface will pan input 1 left and input 2 right. If you want to record stereo, you record to both inputs and then pan them in the daw after recording.