How to use one monitor with two computers?

I recently found myself confronted with a request from two users that made me wonder... The situation is this: they both have their own working place with two monitors each (which they mainly use for CAD). They sit opposed to each other. The thing is that every 10 minutes one of them calls the other one to come over to his desk and look at something on the screen. Now they asked me if there was a way to get the other user's desktop on one's own monitor with a simple click or switch. Of course, there are things like VNC, RDP, TeamViewer etc., but that is not nearly as performant as it needs to be in this CAD and 3D context.

Now I wonder if there is a way to achieve what they asked for. A valid alternative would be another big monitor that gets installed on the wall and that can be controlled by BOTH users at the same time. And I'm not talking about some KVM switch solution, because they both want to use the screen at the very same time.

So I'm really excited to see if there is a way to get things up and running like desired? Thanks for your help on that issue!

If the monitors support support multiple inputs, you could achieve this by feeding the video from User A's computer to the alternate input on User B's monitor. Let's say their current setups are dual head HDMI, leaving a VGA input free on the display. You then add a VGA connection from User A's computer to User B's display. So, when User A wants to show something to User B, they'd be like "Hey, can you look at something" and User B would toggle their display to VGA, and see what User A has up on their screen. Note that User A would have to make sure it's on the proper screen to share.

This VGA connection could be direct from the User A's system, and you enable mirroring on the third display.

A simpler way without needing a three display support would be to use a combination of a video splitter and adapter (e.g. a 1x2 HDMI splitter, and then an HDMI to VGA adapter to connect to the monitor). That's likely going to run you close to $40 or more for each setup. Again, the user would need to toggle the input on their monitor. Depending on the displays and quality needed, you may want to stay strictly digital, so HDMI to DisplayPort may be needed.

Alternatively, hook a large screen TV (or any display that takes multiple HDMI) up to use as a display they can both seen (there are those el cheapo 4k TVs now, though the refresh rates are low), and have the split HDMI input go to the TV, and toggle the TV to use input 1 or 2 depending on which user wants to show something.

Oh, and if the displays don't support multiple inputs, you could fake that as well using an A/V switch. Basically devices that take multiple A/V inputs and let you select which video to output. You use to see these a lot back when RCA cables were dominant, as lots of TV equipment didn't have multiple inputs. You can still find them for VGA, HDMI, etc.

It seems like you would just want a KVM switch, which will take inputs from both computers and use a switch to decide which output to display. It would be cheaper than buying another video card and more performant than VNC

Unsure if the graphical quality would be up to snuff, but perhaps some type of collaborative software is all that is needed. So something like Microsoft Lync which allows them to start a chat and share their desktop to each other. Lync is the one I am familiar with, but I am sure there are other similar applications out there to provide the same functionality for impromptu sharing.