The latest version of Luna Display now allows Mac users to extend their display to another Mac computer. The ‘Mac to Mac’ mode offers the same kind of feature as with the SideCar for the macOS Catalina, allowing you to get more screen space to work in.

The company claims that they’re different from Apple in that they create software that makes use of several Apple products in conjunction.
To pair up an iPad and Mac using Sidecar, you’ll need to update to macOS Catalina. The feature is also restricted to newer models. Luna 4.0 supports older models and can work with macOS El Capitan on the main Mac and Mountain Lion or older on the secondary display.
Running Luna requires a dongle that must be plugged into the primary Mac and an app that must be installed on both computers. Both must be connected to the same Wi-Fi network as well.
Luna 4.0 is priced $69 at the developer website.