You find the twins on the fifth stage, third section. They can hit hard and have some tough team-attacks, but they thankfully give enough notice that you should be able to avoid them.
The most important thing to know here is that if you accidentally down one of them instead of both, the other will stop whatever he's doing to go revive the fallen one. Simply wait for them to both be going again, and continue the fight until you take them down at once. It's not as tough as it sounds