I think you can edit as many parts at once as you like if you select several parts before opening your editor(s) of choice.
Give it a try…
Hold ctrl while selecting several parts in the main project window, then open the editor(s) you wish to work with.
Another approach that helps insure you’ve got all the parts on a track is to hold ctrl and select several tracks, then right click on the track and choose ‘select all events’. Open your editor(s) once all of the parts darken.
I do this (process different parts on different tracks) with the Logic editor all the time…
As for the graphic editors…I’ve less practice at trying to manipulate several parts at once in the same graphic editor, but I’m pretty sure the tools are there to pull the part you’d like to ‘draw in’ to the foreground…as for the editing existing events…it seems you can grab and manipulate them easy enough no matter which part layer they’re living on. It comes in handy, but it can get kind of messy keeping up with which part gets any ‘new drawn in data’. Practice/trial/error may be required to get the hang of it. Also, you might need to remember to go back and freeze tracks to sync up channels if you’re painting in new data.