Is it possible to set up a grand staff to have different endpoints for the treble and bass staffs? Basically this would be for a synthesizer player with one instrument library loaded for the left hand and another loaded for the right hand. I am not a keyboard/synth player so I’m not even sure of what a player would prefer. Would it be better to notate it using two different players? If so, how would I notate that one player plays both of them? Coming from Finale, I am using the ARIA player. I do have the Kontakt 7 VST, but have used it mostly for building effects instruments, although I did build a calliope instrument once from sound files of individual notes. But that was a lot of years ago. If I used VST other than ARIA with a Garritan library, I would have to search for libraries that give me the sound I want. Looking forward, it could be better to use sounds that can be loaded onto a synthesizer.
I think I’ve answered my own question, outside of learning how to turn a bracket into to a brace. When I created instruments Synth Lead and Synth Brass, both in a single player’s hand, next to each other in Setup mode, they appeared with brackets connecting them, perhaps because they are both electronic instruments. I added music to both of them and, even though they both play at the same time, no instrument change was automatically generated. I thought that there would be an instrument change but am happy there is not. Once I set their endpoints, they both play as expected.
Yes, you can enable independent voice playback for a regular piano / grand staff instrument:
I don’t know if that was intentional or serendipitous; I can say that, much like an organist playing on separate manuals, I have seen keyboardists playing two different synths at the same time! 
— Jim
Thanks Lillie. It’s good to know this.
Probably serendipitous. What I like about leaving it this way is that if one (or both) of the staffs have no content in a flow, they can be hidden. I’m not sure if I can hide an empty staff in a grand staff.