It depends on what permissions and limits the Admin user has given you. There are options, which would allow you to add multiple domains and allow you to assign your own disk space and bandwidth quota per domain. If you cant do this you may not have sufficient permissions set by the admin user.
There is no way of grouping two domains together for a domain user, there are basically three levels in Plesk, you have Admin, Client and Domain User. The admin can give the clinet access to create a specific number of domainswith various limits. The clinet can in turn set up domain users which have access to a single domain with various permissions.
As a client you could set up two domains and set one to redirect to the other, you can even set up a domain user on each of these to allow a user to administer each of them individually but a domain user can not access more than one domain at a time.