This is what the arrangement of keyboards now looks like with my computers. Basically on the left you have the double keyboard slide with two keyboards on slides under the desk, and then a third keyboard above. Over to the right you can see a fourth keyboard on top, and then below it two KVM switches (with four buttons and five lights each) are stacked directly under the desk top.
The way this works is the keyboard and mouse on the big slide at the bottom are connected through the black KVM on the right, and the keyboard and mouse on the small slide above them are connected through the white KVM on the right. Both KVMs are connected to all four computers. the net result is that of these two keyboards and their attendant mice, each of them can be connected to any one of the four computers independently of each other.
The two keyboards that are up on the desktop are each connected permanently to one specific computer. Both of these are multimedia keyboards with a built in touchpad to perform mouse functions. However, for convenience, the left hand keyboard has a separate mouse connected alongside it. This one controls the mediapc and the right hand one controls the win10pc.
Having two keyboards able to be switched around instead of one is quite useful when using two computers to work on simultaneously as the big keyboard can be assigned to the one doing the most work and the secondary computer still has a readily accessible keyboard without having to switch between computers all the time. This is all helped as well by the double slide. Having finished installing that last week the next task was to make the shelf to let the keyboard for the win10pc sit on the desktop instead of making another slide for it to sit underneath, and that in turn cleared the way for the two KVMs to be stacked with special brackets underneath instead.