Hello.
I repeatedly miss this feature (since years!) and want to suggest it:

All of my brochures/booklets are printed with a sheet of thicker paper (120g) for the outside cover and thinner paper (60-80g) for the inner pages.

Currently I have to manually sort the different kinds of paper and put them into one paper tray. This is extremely nasty, costs lots of time, and is therefore problematic when multiple booklets have to be printed.

I'd recommend adding a simple setup (i.e. in the booklet definition window), where I can simply choose one paper source/tray for the first/outside sheet and a different one for all succeeding pages. This is surely the easiest way and will match most requirements.