Chaos book pg. 94 says Shaggoth unit size is 1 so they would have to be deploy separately RAW but that doesn’t stop you from doing something outside of RAW with a buddy in a basement game as long as you agree about it ahead of time.
I would say that making monsters a unit diminishes some of the flexibility though as they are no long a line model with 360 movement you’d need to wheel/reform with them. I think I’d rather deploy 2 right next to each other if I wanted a similar effect but that comes with some limitations too. Interesting to contemplate.