(How) can you use a second laptop as a second screen?
Go over the network.
Usually not as a simple monitor replacement. Laptops usually don't have a slot for external video input.
As a rule, this is not simply possible with a cable. Laptops only have connections for the video output (i.e. HDMI Out etc.). So you can transfer the video signal from the laptop to another screen. What you would need would be an input, and a laptop usually has that. Not.
You could think of somehow streaming this over the internet or something. Of course, that depends on your application, whether that makes sense or how you could best implement it.
Transfer your picture to the laptop would e.g. Via VNC or team viewer. However, you would only duplicate an existing image, not expand it. It would also not be very fluid.
It would be most effective to remove a laptop with a defect, but the screen is still there and remove the appropriate driver board e.g. Order from Aliexpress. Then you have a DIY 2nd monitor. There are some YouTube tutorials.
For something like that you need USB Caption Card, so that it is also possible to transfer laptop screen over other laptop screen.
I would advise against Najja Teamviewer as a screen transmission.
Because 1. The Teamviewer screen transfer ends up on the Internet and can access a laptop screen wherever you are.
Then 2. You don't get good FPS performance, because I was glad that I was able to transmit the cell phone display, that I sometimes managed to scroll properly.
