I won't go into all the math, but here are the jackpot points where multiple winners are equally likely as exactly one:
PowerBall: $975 million
Mega Millions: $1.65 billion
Not that you asked, but here are the jackpots where the probability of at least one winner is equal to the probability of none (50%).
PowerBall: $704 million
Mega Millions: $867 million
This question is raised in discussed in my forum at Wizard of Vegas.