the 270x pulls up to 284W maximum giving you POSSIBLY enough to power two of them off of a 650W supply while gaming (since that wont take the full 284W during gaming) so a second 270x might be ok since you computer should never draw 100% power but IF it did you would probably crash, so i would not recommend doing it, and if you did i would not getting ANYTHING else that draws power for your system.
TL;DR i wouldn't get a second one, just upgrade to a more powerful single GPU in a few years