In a weapon "flashing" scenario, or brandishing scenarios where the person does not seem clearly intent on using the weapon soon, is there imminent danger of death or great bodily harm? I would argue not. In which case, no other recourse besides calling 911 would be legal in most states, imo. We are not the police, let the police handle that stuff.
In all cases, call 911. If you're uneasy that it may escalate, try to monitor the situation safely until police arrive. Don't interfere, don't threaten, don't try to take on a police-like role. There is plenty of time to pull your own weapon should need arise. But if you do, imminent danger of death or great bodily harm better have arisen, or you're just another brandisher. It's also not that great to be one of two people with weapons out when police arrive.
Again, just imo and ianal.