By absolute definition, no. That's why it's called a "Primary" key and there can only be one primary.

In all practicality, yes. You can create a UNIQUE constraint on a NOT NULL column and for all practical intent and purposes it works the same way as a Primary Key. It's what some folks call an "AK" or "Alternate Key".

If this is for an interview, be sure to mention that the PK does NOT have to be the clustered index. You can only have one clustered index.