It is possible, just that Windows Display Control Panel/Settings user interface does not support it.
This applies to spacedesk and all other displays.
You can mirror screen 2 to all your iPad screens using spacedesk Driver Console with Videowall mode enabled.
Just follow the workaround steps below:
Let’s say you have these displays on your setup:
screen #1 – main display [1920×1080]
screen #2 – spacedesk iOS viewer [1920×1080]
screen #3 – spacedesk iOS viewer [1920×1080]
screen #4 – spacedesk iOS viewer [1920×1080]
1. Open spacedesk Driver Console and enable Videowall mode
2. Assign all clients connected to Video Wall.
— You will see on Connections -> Active (3 connected), 3 iPad clients connected, click one of them, then click the Videowall tab in the right panel
— In Wall Index box, enter any value e.g. “1”, then it will create a Video Wall1 underneath CONTROL
— Then right-click the other 2 clients below, then select “Assign to Wall1”
3. Change dimension of Video Wall1, by clicking it underneath CONTROL, set the Size (Width:Height) to 1920 : 1080