Unfortunately, SpaceDesk requires that at least one of the computers has a different version than Windows 10 installed.
If you are using Windows 10 on all the computers you need to connect, you may find a solution by installing Microsoft Garage Mouse without Borders, a free tool which allows you to control up to four computers from a single mouse and keyboard.
By using it I succeeded in having three work areas instead than two; more specifically, I have my main laptop connected to an external monitor phisically, then I connected it to my tablet by using Microsoft Garage Mouse without Borders and I use it as a third external monitor. They both have Windows 10 installed on them.
You may do the following:
1) download Microsoft Garage Mouse without Borders from here. You will install it on all the computers you want to connect (in my case the main laptop and the tablet);
2) Install and run it on the secondary device first (my tablet, in my case) and follow the instructions; it will ask you if you've already installed it on the other devices, you have to select "No";
3) an ID and a password will be shown at this point: you want to save them for later;
4) Run and install it on the main device (my laptop, in my case);
5) When it will ask whether or not you've already installed it on the other devices you want to select "yes"; you have to enter the ID and the password previously shown on the previous device.
6) the devices are now connected: you can now copy text or drag and drop files across computers.