while mining granite for a keep(which was instantly deconstructed by a griefer.) i was using a pick in my hands to tunnel and 1 in my inventory to shape ... when the durability on the mining pick i was shaping with hit 0.00 i was still able to shape with it indefinately. the picks in my hand however would die n id have to swap it out with a new one.
Thanks for checking this post and good luck with your fixes.