As far as I am aware, you cannot add two external drives at the same time. However, you can alternate between two or more drives if you want more space.
Is it possible to add 2 external hard drives to a dvr box?
No, if you mean can one run two (or more) drives simultaneously off the same DVR. However you can add an unlimited number of external drives to the same DVR, if you want the change them out and run each drive alone, one-at-a-time. Just remember that each drive will be "married" to your specific DVR and can only play back recorded content on that specific drive.