multiple admins logged in at same time

Are there any restrictions if more than one admin is logged in, such as the inability to edit a page?

There is only one admin, if you share the admin user you can expect issues if 2 or more are working on the same thing.

If using the assignment of tasks, you will have less possibilities of overlap

If Person 2 logs in as the admin when Person 1 is already logged in as the admin, then Person 1 will be logged out.  This is because a new session key (stored in a cookie and in the db) gets generated when Person 2 logs in.  The new session key effectively invalidates the session key used by Person 1, which is what causes the log out.

We only ever store one session key for admin logins.  This is why only one admin can be logged in at one time.

The session key is just a random set of characters that is used in lieu of a password after the initial login where the password is actually provided.