Dear all,
finally I was able to test your suggestion during the weekly rehersal. Despite panning the guitar to right L1 system and the voice to the left one, the sandwich effect was still there. Nothing changed. Also tried to lower the mixer output volume and increase both L1 volumes, lower both mixer and L1, raise both volumes....
Long story short, I tried everything without any result at all .
After one hour testing, desperate, I decided to change the input channels in which Kemper and Mic were connected, using slot 4 (Mic) and 5-6 (kemper, stereo). Magically the problem was solved . I still have to pan the guitar and the voice to different L1s but now it works, no sandwich effect at all. Found out later that the first channel of the mixer (Sonic Station 16) was responsible for the problem, which is quite strange since the clipping control did not detect any anomaly. For some reason, the Mic connected to the first channel was causing the L1 clipping (I discovered the issue watching the input led of the L1 during one song. It constantly became yellow).
Anyway, I notice that this PA (Bose L1, mod. II) is very sensibile to input volume. You have to set the mixer output lower than other PAs to avoid clipping.