[Luc]

Can you use modifier keys spanning the two keyboards? E.g. shift on the left, and a letter on the right to type a capital?

[crazygringo]

Yup. Though I think I may have had to install (free) Karabiner to enable that:

https://karabiner-elements.pqrs.org/

So I'll e.g. page down by holding Fn on the left keyboard and down arrow on the right.