Some complicating factors are:
- yes, pointing precision will be better on a (hypothetical) 15.6" NexDock screen as compared with a 5.7" Librem 5 screen
- if you have an adaptive app that displays and behaves differently on the big screen as compared with the little screen then it may be tricky to use the little screen for pointing (still OK to use it to move the cursor though - and you probably want to turn the little screen off, while still generating touch events - is this possible?)
- it would be trickier still if you chose to use the two screens as separate screens i.e. of the three modes (independent, appended vertically or horizontally, mirrored) perhaps only mirrored makes sense for what you are suggesting, or the little screen is just off.
Still, at the end of the day, these are all technical issues - and those would be trumped by other issues e.g. is there a market demand? is it financially viable? does the engineering bandwidth exist to make this product? (taking into account other products that could be explored)