Dual-class

Dual-classing rules in the books are very light. This page solidifies them a bit.^Ex We haven't properly playtested dual-classing yet, though we expect to have some dual-classing happening soon. We'll solidify the rules a bit by then.

Characters can only dual-class after reaching 2nd level or higher in their current class. Therefore, a character cannot dual-class at character creation.
There is no additional downtime or training time to switch classes.¹⁾

Note that during the “catch-up” period (where the new class level has not exceeded the previous class's level), absolutely no functions of the previous class(es) can be used except for HP, including to-hit tables, saves, powers, weapon proficiencies, and so on.²⁾ If a function is used, no absolutely no XP is earned for the current expedition.³⁾

¹⁾

We are not 100% settled on this, but it is a fine application of Occam's razor. In-world you can imagine that the character had been preparing to dual-class for some time, whatever shape that takes.

²⁾ , ³⁾

PHB 33.