Complex sentence: Utah is the only state besides Nevada to have ever used the firing squad, although executions of this type are authorized in Oklahoma in the event that lethal injection, nitrogen hypoxian and electrocution are all declared unconstitutional.
Split into simpler sentences: Utah is the only state besides Nevada to have ever used the firing squad. Oklahoma is only other state currently allowing firing squand, and solely in the event that lethal injection, nitrogen hypoxia and electrocution are all declared unconstitutional.