This very much depends how you use that second line of credit and what your current credit is.
- If you max it out and then do not pay then it will hurt your credit.
- If you max it out and pay the minimum but currently have a low credit score it will help. If you already have a 700+ rating then it will hurt it.
- If you Never use it then it will have little impact on your rating but could cause problems if your available credit exceeds 50% of your income.
- If you use it regularly but pay it off every month then it should improve your credit rating.
There are of course many more combinations buy you can probably infer the impact based on these cases.
Your credit score is based on your likely hood of being profitable to a creditor should they issue you credit. This is based on your history of your ability to manage your credit. Having more credit and managing it well shows that you have a history of being responsible with greater sums of money available. If you use the card responsibly now then you are more likely to continue that trend than someone with a history of irresponsibility.
Having a line but not using it is not a good thing. It costs the creditor money for you to have an account. If you never use that account then you are not showing that you can use the account responsibly so if you are just going to throw the card in a safe and never access it then you are better off not getting the card in the first place.