handwriting distorted in dual screen mode

The writing is distorted because your tablet is configured to map the pad area (which is a rectangle of given aspect) to the whole display surface (which is a rectangle of a different aspect ratio once you add in a second screen). To avoid this, you need to map the tablet area to just one of the two screens. This is the job of your system's configuration scripts, not xournal's.

The magic command (which some distributions apply automatically in some use cases, but usually that is only for tablet PCs, where the display surface itself is the tablet area), is something like:

xinput map-to-output <device id> <display name>

To find your device id, "xinput list" -> find where it says "Wacom ..." (you may find that it will be enough to map just one of the wacom devices, or that you need to do it for each of them using several xinput map-to-output commands). For example on my system, xinput list gives among others:

| ↳ Wacom Pen and multitouch sensor Finger touch id=9 [slave pointer (2)]
⎜ ↳ Wacom Pen and multitouch sensor Pen stylus id=10 [slave pointer (2)]
⎜ ↳ Wacom Pen and multitouch sensor Pen eraser id=16 [slave pointer (2)]

To find your display name, "xrandr --listmonitors" -> will give the names (and resolutions) of all the monitors. My laptop's display panel is called "eDP-1". So:

xinput map-to-output 10 eDP-1

is the command to map the pen's logical input surface to the display panel.

If you want to fine-tune and map your tablet to something more customized than a whole screen, the relevant property is the Coordinate Transformation Matrix

xinput list-props <id> gives, among other entries:
Coordinate Transformation Matrix (148): 1.000000, 0.000000, 0.000000, 0.000000, 1.000000, 0.000000, 0.000000, 0.000000, 1.000000</id>

(Right now this is the identity transformation, since I'm mapping my whole tablet surface to my whole screen -- I don't have my external display right here to show what it'd become after plugging in the second display and mapping to just one screen, but you can probably test yourself.)

The most relevant entries, unless you have a rotation issue, are the first (x scaling), third (x translation amount), fifth (y scaling), and sixth (y translation amount). So e.g. if I want to map the whole tablet to the upper-right quadrant of the display, I'd use:

xinput set-float-prop 10 148 0.5 0 0.5 0 0.5 0 0 0 1

(If you get yourself completely lost in the coordinate transformations, use map-to-output to reset the settings...)

The gnome settings for wacom tablets are rather limited and, in my version at least, there is nothing useful to help with this sort of screen mapping in the graphical interface (ideally this is where one should be able to do it) -- but in the meantime, the "xinput" command-line is your friend (somewhat).

Hope this helps,
Denis

Thanks for the very detailed instruction. I will follow the procedure and give a result description. I think mapping the tablet to only one of the available displays will do in my case but I'll keep the fine tuning in mind.
Since this isn't a bug and a very reasonable explanation including a fix is available I think this thread could be closed.

Edit: I implemented the suggestion to remap the tablet to one screen and it works perfectly after that.