This is what we know:
"The other thing we've changed is how magic users use spell slots, making it less punishing to level more than one magic class. One issue with multiclassing is that if you multiclass early in the game, you don't get strong abilities like "Fireball" at the same level as a "pure" class, but we wanted players to be able to multiclass from the start of the campaign, without having to necessarily wait for the advanced levels, so it was the case to revise the use of resources a bit."
The recent Community Update #21 didn't mention any additional specifics, or even confirm the above.
