I often shoot with 4-6 Yongnuo YN565EXII flashes, each on its own YN622c trigger (2 per umbrella for key and fill, maybe 1-2 more on the backdrop). I'm using a 5D3 and at most venues, I have no problems with these triggers at 1/200. Many times I'll drop it down to 1/160, but I like to start at 1/200 to test the reliability of the triggers.
There is one venue, conference rooms at an ice rink, that will sometimes give me problems and I need to drop it down to 1/125 or play with the channel. This is very odd in that issues I run into seem to be venue related. My guess is that 2.4 mhz Wifi, wireless mic, or other radio signals are affecting the triggers, which operate at the same frequency.
When I do have a sync issue, it rarely affects more than one flash and never leaves me with a totally black bar. I'm curious. Did you have a trigger on each flash or one trigger driving both (or maybe the second on optical slave)?