Oh I though you had raw input off. Can you try disabling it and restarting the tablet driver by completely closing it instead of using the "Restart Driver" button?
When I tried to reproduce the issue I noticed that I got issues with increased horizontal sensitivity if I had either:
- not completely restarted the tablet driver after enabling the second monitor, in this case the problem was on both desktop and osu
- kept osu running while disabling the second monitor and doing a complete tablet driver restart, in this case the problem was only in osu and when raw input was on. The problem did not persist after restarting osu. Seems similiar to what you mentioned.
Hopefully you should be fine if you keep raw input off, avoiding disabling/enabling monitors while osu is running and always doing a complete restart of the tablet driver after disabling/enabling monitors.
If you wanna confirm that you have the same sensitivity with one or two monitors, I think you can just pick one of the grey dots on your tablet, put your pen and it and remember the corresponding position on your monitor. Then try the same but with both monitors enabled and a complete tablet driver restart(with mapping only to main monitor)