Just so you're aware, this machine doesn't have two jackpots (some machines do). The term "double jackpot" is actually a ruse to make punters think they're going to get paid twice. It means there are two winning combinations that pay the jackpot. Sega did make a "Double Pay" version of this series, but this is not it.
What it should have is a working reserve jackpot (if you decide to buy, check the reserve is working). This is part of the jackpot unit which, once the visual jackpot coin holder is full, begins to trap coins in the reserve coin area above the main chute. This means when the jackpot pays the reserve coins fall into the main chute ready for the next big win, so it's actually never empty and encourages the punters to play. These were often removed by operators as they were problematic, but at this price I would expect it to be there and working.