X Windows is a network protocol. You can run an application on one host and have the display appear on another host. In the dark ages, or the mid-1990's, whichever is most recent, I fired up X windows on a pair of computers. Then I fired up a program which ran X Windows on top of that. The upper layer X Windows treated the native copies of X Windows as if they were (slow) monitors. This produced the magic effect, that windows could be slid off of the side of one screen, onto the other screen.
It was like having a modern multi-screen system, except that the only connection between the two computers was the blazingly slow 10Mbit ethernet. It worked fine given the constraints of the systems. I had two Sun 3/50 computers. They were upgraded to a massive 12 Mbytes of RAM each. Normally, a Sun 3/50 could only drive one monitor. These were monochrome, not even grayscale. The software didn't support things like rendering contexts, Open GL, or any other X extensions, but it didn't matter, as neither did the hardware.
My google skills are failing me. I can't recall what it was called. Xpra, xpra.org, looks like it lets you move a window from one host to another host, but it doesn't turn the two hosts into a single large virtual display sharing a single keyboard. With Xpra, you have a keyboard and mouse per display. Xpra, and two Raspberry Pis would be an upgrade compared to a Raspberry Pi and a VT220.
Do any old timers remember the software? Is there a modern equivalent that will work with Open GL applications?
To review: the software let me run two (or more) computers, with one mouse, one keyboard. It would forward my keystrokes and mouse events over the network, to the other computer, when I slid the mouse off one screen, onto the other. Windows could be half on one monitor, half on the other. If the CPUs weren't too busy, it wasn't even very painful. (Each Sun 3/50 68020 was rated at 1.5 MIPS. A Raspberry Pi should be about 200 times faster.)