We have a previous spacedesk users that verified that MS Flight Simulator does NOT support/recognize touchscreen input.
And according to that user, it can only recognize mouse input from a remote display.
We can only suggest either of the following:
1. Use mouse touch which automatically activates on your second viewer connection.
Current spacedesk’s touchscreen only supports in first spacedesk display, so on your first viewer app just activate touch screen first, then connect a second viewer app where you can move your flight simulator app and try to send the mouse touch input from there.
2. Select relative touch input in your viewer app settings (the touch input will behave as mouse pad for your primary machine).