When a user has 2 conflicting Roles assigned, Marketo will, more often than not, be more restrictive in its willingness to let the user perform the action if the other User Role does not have the permissions to do the action.
Marketo Support often sees users assigned the Admin Role, as well as a Marketing User Role. The Admin Role has permission to do everything. Adding an additional Role can cause restrictive permission errors when that user is attempting to do something that the Marketing User Role does not have the ability to do.
Example: A new User is assigned 2 User Roles each with different permissions. One User Role has the ability to approve email assets, and the other User Role does not have that permission. In that case, Marketo may restrict the User from being able to approve the email asset.
There is not necessarily a limit to how many Roles can be assigned to a User. There is nothing prohibiting you from assigning multiple Roles to a new User. It is recommended that Users are invited with only one role assigned. The resolution could be to tailor User Roles for the specific person, or job position in mind.