The modern alternative would be the combination of an HDMI switch and a USB switch (aka a KVM), but the software-hardware comparison remains equally valid. Multiplicity can emulate the capability of the KVM switch and let one display serve all the connected computers. Multiplicity comes in two versions the standard Multiplicity has the ability to copy and paste images and text between computers, while Multiplicity Pro can control up to nine client computers and can copy files, folders, and other data between machines. Switching is triggered by movement of the mouse to the appropriate side of the screen (or keyboard shortcuts, if desired), both from the desktop and in full-screen video modes. It is closer in concept to a KVM switch, but while these have multiple cables to each computer, with Multiplicity the keyboard and mouse remain connected to the host computer and input is forwarded from the host to client machines via network connections - typically over TCP/IP port 30564.

Heck come to think of it if there is no login/password and it autoruns Barrier at start it may just connect automatically if none of the settings/ip address have changed.Multiplicity is unlike remote desktop applications in that instead of opening windows to a client computer on a host computer’s desktop, the mouse pointer and keyboard focus shifts from one computer to another. If it's not crucial for one of the computers you are using to have a login/password maybe could set it to just start without needing to login? Then you wouldn't need the keyboard and could just use a logitech mouse like I have to click the buttons to get the software running. My main mouse is a logitech bluetooth unit that has a button on the underside that lets me switch between different computers so I can use that as well and may be something worth looking into ( MX Anywhere 2S is what I use). I can also use the trackpad on the macbook until I get the Barrier software running in the morning. With the main iMac it has the keyboard plugged in and the macbook already has its keyboard so I haven't had to figure that out. Not sure if you could do something like that.

One of the iMacs (older 2013 model) is just a second monitor to the first one and would also need a mouse/keyboard to start up but I rigged it some time ago to login without needing username/password and autorun the program that lets it act as a second monitor at startup so I haven't had a mouse/keyboard plugged into that one for a long time. That is one problem that would still need to be worked out I'm afraid.

Sharemouse connect to another pc