Using more than one Kontrol S MK3 keyboard simultaneously on the same computer (e.g. Kontrol MK3 and another Kontrol MK3 / MK2 / A or M keyboard) is not supported.
This can lead to inconsistent communication between your computer and the keyboards and cause a number of instabilities.
If you want to use two MIDI keyboard controllers simultaneously, it is possible to use a second non-Kontrol MIDI keyboard controller, as this should not interfere with the Kontrol integration.