The video below was tweeted by BLM Idaho July 13, 2019 but they did not say when or where it occurred. It may have been at the Ridgeline Fire 5 miles northeast of Albion, Idaho the same day.
— Bureau of Land Management Idaho Fire (@BLMIdahoFire) July 14, 2019
It is interesting that the lead plane was much lower than the DC-10. If the tanker had dropped at that height it would have been an extremely low drop and the retardant would have been rapidly moving forward when it impacted the ground. Maybe the lead was low to give the following air tanker pilot a better 3-D perspective of what piece of ground it was over when they released smoke or said “start here”, and they have already agreed on the drop height.