You can purchase additional returns by following the steps as mentioned in the article - https://turbotax.community.intuit.ca/questions/1189885-purchase-additional-returns. Please follow the Desktop section.
I hear you have already purchased another desktop version. You will have to install the product on another machine where you had not installed the earlier purchased TurboTax.
You cannot install two different TurboTax on the same machine with different keys.
Hope this helps!