These are two different models so this means it are two seperate calendars.
A new item created under Projects > Tasks will not come under Calendar and items from Calendar will not come under Projects > Tasks by default.
You could however add code so that when you create a new task (or calendar event) that there is a new record created on the other model (calendar or task).
An Automated Action (watch for tasks to be created) that runs a Server Action (create a matching group calendar appointment) may be a good way to do this. Check the demo database for the Automated Action "Set Auto Followers on leads which are urgent and come from USA." to get started on understanding how this might work.