WordPress is a free and open-source content management system written in PHP and paired with a MySQL or MariaDB database. Features include a plugin architecture...
You can if your chosen plan supports multiple websites. Before adding a second site, check the limits for domains, databases, storage and processing resources. An additional domain that displays the same content is not the same as a separate website.
For easier management, use separate directories and databases, along with different passwords. Websites on the same account may share resources, and a security issue could affect more than one installation, depending on how they are isolated. If the projects have different requirements or belong to different clients, consider separate hosting accounts.