First off, players from both games have to work together to achieve cross-milestone rewards. To earn them, The Alchemist Code players need to complete the corresponding milestone targets set in Brave Frontier and vice versa. Additional rewards such as unlocked dungeons, tomes, and summon tickets will be given after players achieve the first, third, and final milestone targets.
Both sides will be able to get new characters too. The Alchemist Code players can summon Brave Frontier's Selena, a swordswoman who's also one of the Six Heroes. If you finish the Brave Frontier Event Quest, you'll receive Vargas Soul Shards, which can be used to get Vargas, another one of the Six Heroes in Brave Frontier.
Meanwhile, Brave Frontier fans will get Chloe, commander of the Scarlet Flame Imperial Guard of Envylia and Lofia in The Alchemist Code. You can also unlock Logi, Squire of the Blue Flame Guard, by clearing the Vortex Dungeon: Love Never Fades.
Lastly, the crossover event offers login bonuses for both games. The rewards get bigger as more players log in, so you if you want the good stuff, be sure to check in daily.