Mosaic creates a single desktop from multiple displays and GPUs. To activate this mode, select the menu item "Set up Mosaic" . Click on "Create new configuration"  to start the setup wizard.
Define the number of displays , the number of rows and columns  and the orientation of the displays . Click "Next".
Select the displays to be used , the resolution  and the refresh rate  of each display. Click "Next".
The single displays must be arranged to the combined mosaic display. The corresponding numbers are shown on the real displays and should be arranged according to your setup. By simply dragging and dropping  the display areas can be placed. After placing all displays, click on "Apply" button, which will appear. This may take some time, please do not stop this process.
For further information about how to set up a mosaic please also have a look at the official documents from NVIDIA
How to set up a mosaic
NVIDIA Mosaic Technology