You can set what the remote Raspberry Pi computer boots to, and start x2x. X2x is similar to a KVM-switch, but with several screens and copy and paste between screens. You select active computer by moving the mouse-pointer to its screen.
There is a blog post in Swedish (på svenska) about x2x: http://enradare.orbin.se/#post66
These scripts assume the left computer has the mouse and keyboard connected. But this can be changed in
the scripts on the computer with keyboard by changing east to west, north, or south.
On both computers do:
cd
git clone https://github.com/mobluse/x2x.git
The computers must be connected via network with port 22 for SSH open.
~/x2x/boot2x2x.sh raspberrypi2
~/x2x/startx2x.sh raspberrypi2
~/x2x/boot2console.sh raspberrypi2