Yes, up to 4 Intel Unite clients can share their screen at the same time their own screen. They won't be able to sync up the content as they are independent streams.
The bidirectional annotations is possible, each client will need to open the "View Presentation" window after they start presenting, this will allow them to annotate on the content being presented by each user independently.
In order to be able to present content an Intel Unite Client should enter a PIN displayed on the particular Intel Unite Hub where the client wants to start sharing, the client can only connect/share to one hub at the time.
An Intel unite Hub cannot connect/share to another Intel Unite Hub.
Please, let me know if this clarifies your question.