Sorry, you are unable to install two copies of TurboTax for the same year on one computer. You are only allowed to use one Installation Key Code (IKC) per computer, and file 20 tax returns with that IKC. 

 https://turbotax.community.intuit.ca/replies/2647908

can I install a key code from a second disc to the original download to receive 20 returns with 8 over $25,000.00?

In order to add more returns to your Desktop version of TurboTax, you will need to upgrade your version- since you are only allowed 20 returns per computer. With the code, you entered it equals 20. 

This is your only option - https://turbotax.community.intuit.ca/community/product-information/help/how-do-i-upgrade-to-a-differ...