Hooking up an in-ear with the Kemper works very well in my experience but can be a bit challenging to start with as, due to the Kemper's flexibility and many outputs, there are several routes you can go down. Here is what I have tested and found to work well for me: I always run my KPA on top of and via a 2x12 cabinet no matter if for rehearsals, club gigs without PA system, or larger venues with full FOH support. Nonetheless I also always use in-ear monitoring connecting the headphones out of the Kemper via a mono cable with the left channel of my LD Systems GMEI while receiving a blend of the band sound (minus guitar signal!) on the right channel from the mixing console. Here I have also used an ambience microphone instead for smaller gigs where not all instruments were mic'ed.
Running the LD Systems in its focus mode (dual mono) allows you blend these two signals (Kemper and band sound) and adjust volumes separately and on your own as needed directly on your body pack.
While running a mono cable in the (stereo) headphones jack may not sound very elegant at first sight, I was told by a technician that it cannot hurt the Kemper and it seems to be the only option left when main and monitor out are already "taken" (for obvious reasons I run a pure amp profile without cabinet on the monitor line). Also this gives you access to the enhanced sound of the headphones section of the Kemper - even though in mono.
I have been successfully using this set-up for quite a while now.
Hope this helps.