I have most of the time had a multi-monitor setup and when I recently got rid of one of the monitors I started using my laptop as an additional screen to keep documentation on. This works quite well but it gets it gets a bit work to constantly switch keyboard and mouse to control the laptop.
There is a solution for this, a KVM switch. I have not had the best of experience of neither hardware nor software switches. They must have improved since the last time so I started looking for what was available.
During my quest for the optimal software KVM for me I stumbled across Mouse without Borders. It set it up on my desktop and laptop and it has been working great for more than a day! 🙂
If you are using only Windows (10 in my case) on your computers it will probably get you started and if you notice you are missing a feature some of the commercial solutions may be a better option.