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⁾

Upkeep

Upkeep for dual-class characters is based on the level of their active class. In any month that the character acted as another class, that class is used for upkeep instead if the cost is higher. For example, if a F7/T1 slipped and acted as a fighter (or intentionally fell back to their fighter abilities in an encounter in order to avoid death), the base upkeep for that character for that month is 700 gp, not 100 gp.

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.