A:Answer If you are talking about a 2012 MacBook Pro 13" or 15" (17"???) NOT RETINA, then yes. In fact you can swap out the optical drive using an adapter [https://eshop.macsales.com/shop/internal_storage/Data_Doubler] to put in 2 SSDs. I've done that.
The Retina laptops use "Instead of a hard disk drive, the new models ship with a solid state drive housed in a proprietary flash module design rather than a 2.5" notebook drive." [https://en.wikipedia.org/wiki/MacBook_Pro#Second_generation_(Unibody)]