The Paradox Brothers are obtainable through the event "Paradox Brothers: Guardian of the Gate!" which works similarly to the other Legendary Duelist character unlock events. The event's first appearance is May 24 -31.
Para and Dox, are brothers who duel in a tag team format that worked as eliminators. They were hired by Maximillion Pegasus in the Duelist Kingdom tournament, and ended up dueling against Yugi and his companion. It was also the first tag team duel in the anime/manga.
How to get
Elements Unite! You will start the duel with 500 Life Points and your starting hand will be 1 card, which is "Gate Guardian". In addition, you will start the Duel with Suijin, Kazejin, and Sanga of the Thunder on the side of your field. Their effects are negated and they cannot attack as long as they are face-up on the field.
Restart Can be used after starting hand is distributed. Shuffle all the cards from your hand into the Deck. Then draw the starting hand again
Thunder, Water, and Wind Can be used when you life pints decrease by 1800. Randomly add one "Sanga" "Kazejin" or "Suijin" to your hand (these cards are not drawn from your deck). Can only be used once per duel
Three-Star Demotion Can be used when your Life Points are 1,000 or below. Until the End Phase, reduce the Level of all monsters in your hand by 3. This skill can be used to "Suijin, "Kazejin", and "Sanga of the Thunder," regardless of your remaining Life Points. This skill can only be used once per turn.
Labyrinth Builder Shuffle 2 cards from your hand into the Deck to create "Labyrinth Wall" on your side of the field. This skill can only be used once per Duel.
Guardian Call Can be used when your Life Points decrease by 1800. Randomly add 1 "Guardian" card from your Deck to your hand. This skill can only be used once per Duel.
I used the legendary warriors pre-built deck, and as long as you pull a Marmiting captain in your opening hand (or second turn) and his ability special summons a monster its a guaranteed quick victory.