I just got a product key for Microsoft Windows 8.1 through the MSDNAA (DreamSpark) as a student.
Am I able to use this product key multiple times, on both my computers, or just once?
Yes & No
Technically you are within your fair-use rights to activate it on multiple computers THAT BELONGS TO YOU.
Nevertheless, if you install it on more than 3-4 computers or re-activate it on the same computer multiple times (likely 3) after re-installing Windows, you will probably have trouble activating it the next time you want to.
At that stage, you might have to speak to a Microsoft Rep.