Just adding some more info, for future visitors. May or may not be useful. I bought these SB-800 speed lights (used, fairly inexpensive) thinking that, since I already had one in the back of a drawer (I've primarily done ambient stuff, landscape over the years), it would be an inexpensive way to get short duration flash for the particular thing I'm trying to do (combine continuous light with flash to get motion trails and a frozen dancer in the same shot). My thinking was that if I had multiple flashes, I could get enough power, and still get a short enough duration.
If I had it to do over again? I would just purchase a single studio flash capable of this, which I think can be had for about 650 bucks. Even three speed lights are just barely enough power to freeze my subject only a few feet from the flash (reflected in an umbrella). If I wanted to capture a dancer mid leap with the speed lights 10 feet away, there wouldn't be anywhere near the power I would need to get a short enough duration to freeze the dancer (as far as I can tell).
Moreover, as Merco_61 pointed out, the speed lights don't behave as expected with the newer camera I have. I did more experiments today and discovered that, despite the Nikon site saying that the z7ii is fully compatible with the SB-800, several things simply don't work the same, and it's caused me a ton of frustration over the last week because I'm pretty new to using flash at all, and when things don't behave as expected that makes it difficult to learn. Turning off pre flashes is one example. I've discovered a couple others.
Anyway, I've dialed back my expectations for this shot. No mid air leaps, movement landing a few feet from the flashes. I can fire them in manual, and it works. I'll save my pennies for a more powerful studio flash with a short duration as I imagine capturing mid air leaps.