One drop.
Those log flume aren't synchronised to drop at same time, that was a coincidence. It was the blocking system stopping the boats from getting too close to the one in front. It's standard and happens a lot on Fabbri / Reverchon models like the one at IX. Often happens on Brighton Pier's Flume which is the same type.
Other manufacturers of log flumes will do this too if boats get to close, but this rarely happens. In Merlin theme parks I have only seen it stop boats on The flume as they often get too close