I've been looking for ways to do this for months, I'm surprised Squarespace haven't found a solution for this yet.
I sell packages that contain appointments of different lengths, that need to be booked on seperate days. It's very clunky for clients to book a package, and I think it hinders sales when it's not easy to book multiple appointments at the same time.
What I've done with my services is embedded the scheduler on a page that is just for booking that particular package. I've put notes on how to book the appointments. I've also added a thumbnail image to the service that shows what appointment number it is.
My clients still need to repeat the process multiple times to book their appointments, but it's better than nothing.