Dual-class

Dual-classing rules in the books are very light. This page solidifies them a bit.Ex

Characters can only dual-class when they have enough experience to level-up. Therefore, a character cannot dual-class at character creation.1) I highly doubt character creation dual-classing was intended to be allowed. For one, fighter → magic-user dual-class for extra starting HP wouldn't have a downside, so it's unlikely it was intended to be an option.

Characters do not need to pay for the training for the new level they have the XP for. Actually gaining the new level is optional. Of course, the downside of not training is that they will remain be their current level in the initial class.

Characters must pay 1500 gp for 1 special month of training time in order to acquire the first level in their new class.