Why do you want to use more than one file for this? That can really complicate things and is unlikely to be the best approach here.
With relational databases such as FileMaker you really do not want to do things such as creating "an individual contact record to contain multiple tasks". Instead you link your single contact record to all the relevant task records and then display them in a portal or possibly a list layout based on the tasks, but with fields from the contact record added to show to whom the tasks are assigned.
And since you may have a given task assigned to more than one person and more than one task assigned to a given person, you'll likely need a "join" table to link the two correctly:
contacts::ContactID = Contact_Tasks::ContactID
Tasks::TasksID = Contact_Tasks::TaskID
Then you can use a portal to Contact_Tasks on a contact record to show all tasks assigned to that contact and you can use a portal to contact_Tasks on a Tasks layout to show all contacts assigned to that task.