The CS1842/CS1844/CS1942DP/CS1944DP supports Quad-Display mode that connecting two dual display KVM as one Quard display KVM，in this mode the two CS1842/CS1844/CS1942DP/CS1944DP one should be Primary, another should be secondary 

This issue is because  CS1842/1844/CS1942DP/CS1944DP is under the secondary mode

Detail information could refer "Quad-Display (DCC Mode)" of manual of CS1842/CS1844/CS1942DP/CS1944DP that can download from :

CS1842/CS1844:

https://www.aten.com/global/en/supportcenter/downloads/?q=CS1842

CS1942DP/CS1944DP :

https://www.aten.com/global/en/supportcenter/downloads/?q=CS1942DP

Anyhow, please make sure the DCC switch on the rear side of CS18242/1844/CS1942DP/CS1944DP is on "P" (Primary) then your device should work fine. Better change the DCC switch by follows step:

Remove KVM power --> setup KVM mode --> power on KVM.

If the issue persists, please contact our Tech Support for assistance.