Correctly (in terms of aspect ratio) map wacom graphics tablet to two monitors

I am using a wacom bamboo graphics tablet in a dual monitor setup on Linux Mint 19 Cinnamon. My combined screen has a significantly different aspect ratio then the graphics tablet. When I draw a perfect circle on the graphics tablet the mouse on the screen draws an ellipse. **How can I map my graphics tablet to both screens preserving a correct aspect ratio?** For a correct aspect ratio a part of the tablet has to be dead. The following illustration shows my current mapping on the left, and my desired mapping on the right. In the cinnamon system preferences, there is a »keep aspect ratio« setting that only works for single monitors. I can enable »keep aspect ratio« as long as »map to single monitor« is enabled. When I disable »map to single monitor« then »keep aspect ratio« automatically turns off too.