AutoIncrement non-primary key or Make unique composite key?
As Vérace suggested, you can do both: create a surrogate (auto-increment) key to simplify joins and related table structures, and add a unique constraint on the two current PK columns to continue to enforce the proper data integrity.

In SQL Server, multiple columns can participate in a unique index / constraint:

ALTER TABLE dbo.Orders
ADD CONSTRAINT UQ_Year_OrderNumber
UNIQUE ([Year], OrderNumber);


Thus there is no need to create a computed column to combine the two columns.