Hi @alion,
You can try 2 things:
1. You can try to connect a virtual display without secondary machine connected using local host connection via HTML5 Viewer.
Just open a web browser on your primary machine (e.g. Google Chrome, MS Edge), then go to http://viewer.spacedesk.net/ and connect to your local machine’s IP address.
Please take note that only 1 connection is allowed when using spacedesk HTML5 Viewer.
2. Enable Videowall and set the Videowall Disconnect Delay to -1 (equivalent to infinite) in spacedesk Driver Console Window.
With disconnect delay set to infinite, it will not unplug the virtual display in your primary machine even after disconnecting the viewer app in your secondary machine. It will only unplug upon spacedesk server switch (OFF-ON) or machine reboot.