Once you've completed the process, it will take two weeks for Epic to transfer your in-game items and Vbucks to your primary account. For Battle Royale mode, the game will merge all cosmetic items, and Save the World mode will retain all llamas, defenders, heroes schematics, survivors, XP, and evolution and perk materials gained over time.
Be warned, though: A few things won't transfer over from your secondary account. Items from the Unreal Marketplace, Creative Islands, and your Save the World account level and progress will not be moved to your primary account.