If you buy jointly, you will both be classed as owner occupiers or investors adn will pay stamp duty if applicable at the appropriate rate.
This is from Revenue
What is the position where the purchase monies are not provided entirely by the first time buyer?
To qualify for the relief the entirety of the purchase monies, including any borrowings, must be provided by the first time buyer. Any person, who provides part of the purchase monies or who is a party to any borrowings relating to such purchase, is also regarded as a buyer of the house and the relief will not be available unless that other person is also a first time buyer. …
mf