It's like parallel universe, both games will exist side by side and you can travel in-between them on same character.
Alot of things WILL NOT transfer over - only says it shares AC and SG currency system (says nothing about the items you get from them). Weapons and units can carry over but functionality may be different... temporary
Confirmed will NOT transfer over:
Character Level, EXP, Skills
Consumable Items used for battle, growth, and enhancement