which one is more or less flexible and which one has or doesn't have (etc.) a tendance for the 2 sides of the nib to cross over each other under pressure?
you got my attention with this post
Discussion
I will compare the fine Lamy nib to the Sheaffer fine nib Since they're closer in terms of writing experience than these two are.
Neither of these are flex nibs so it's not recommended that you apply a lot of pressure on them to spread the tines as that's not how these are really supposed to be used.
That being said, there is a bit more play in the Lamy nib than the Sheaffer. The Sheaffer is stiffer, but writes a tad smoother than the Lamy.
I have not had any tines cross over each other because I'm not pushing down hard on any of my pens. That's more of a calligraphy pen feature than a fountain pen one. A fountain pen should write under its own weight and really only the softer nibs (14k, 18k gold, flex nibs) have any kind of play in them and even then, it's not much.
Basically, if you're pressing down hard on these, you're using them incorrectly and you're going to damage your pen.