That depends on A) What you want to use the second internal drive for, B) what the original is, and C) What is the make / model of your system.
What are you trying to achieve? I can't make any sense out of the question unless someone gave you a second drive and you want to use it.
You can use it for additional storage.
If you clone the original you can run your system from it, only useful if the new one is bigger or faster.