One would expect that either only the section punched would crack, or both would. However, this varies from block to block, rather than being standard.

• Bed: only the targeted half cracks, although both break
• Door: only the targeted half cracks, although both break
• Piston: only the targeted half cracks, although both break
• Chest: both halves crack, but only the targeted one breaks

I can't test double plants since they break instantly.