If you get a dog killed because you didn't have a back up dog ready...
Well if you have a back-up dog ready, what's the logic behind not sending both? How do you know dog #1 is in trouble before you get right up on the catch? Do you lead dog #2 with you as you walk to the caught pig just in case?
Of the dogs I've personally lost, and the others that I have been witness to, all were either mortally wounded or dead upon our arrival to the catch.