I have a two W-2 documents from the same former employer but one says 2015 and the other 2016, do I use both for this years taxes?

If the two W-2s accurately reflect compensation paid to you in 2015 and 2016, then, NO, you must file each W-2 with its corresponding tax return (2015 and 2106).

You cannot include both of them in your 2016 return.

If you have already filed 2015 and this W-2 was not included, then you will need to file an amended return for 2015.