Marco can't.. well he still in quarantine after all, and also it's not time yet to return to Adastra, because he's not done on his mission on Earth (maybe he will be in new sequel, and also we can play as him again, after the final build of Khemia, that's what I hope) speaking of Alex and Cass I hope not....
Maybe in every scene, we can play both of them, well what meant is "switching" on every scene. That would absolutely incredible..
Or maybe they will give us options, for example:
"Play as Amicus"
"Play as Marco"
"Play as both of them"
(That just a examples of options, that I just thought, if they gonna put options like that on the next sequel we're Marco returned on Adastra, that would be great... If you ask me.