This is a solution to beating the twins and it is a tip to pass the Case File before it (Case 5-2), so first time players do not panic on the whole "GAH! HOW AM I GOING TO GET $1,000,000!!!"
First, find the three gambling magazines.
Gambling 1 is found on the top level of the pool area in Palisades Mall, near the water slide.
Gambling 2 is found on the second floor of Bennie Jack's BBQ Shack in the Americana Casino, overlooking the casino-side dining area stage.
Gambling 3 is located in Security Box #304 at the Uranus Zone Bank. Key #304 is in Fortune Park to the right of the Fortune City Hotel entrance, on top of a wooden awning. To find it, climb up on the white stone block, then jump up onto the unfinished wooden awning. It is sitting to the right at the very end of the awing.
Second, Go to the giant slot machine at the Slot Ranch Casino. Viola!
As for the twins, be sure to bring food (I prefer OJ) and combo weapons (spiked bats are a given, but a Defiler wouldn't hurt) and a shotgun or two. You only need to defeat one of them to win the battle.
TIP: Dodge Rolling (pressing down on the control stick) to avoid their attacks will leave them open for a bit.