From the post:
One of the trickiest parts of the rules is multiclassing. At its heart, multiclassing allows you to build almost any character you can envision, taking parts from multiple classes to build the perfect version of your character. Making these rules play well with the rest of the game, unfortunately, has always been a challenge. Concepts that really should work together just fell flat, leaving you with a character who could not perform at its level and keep pace with single class characters. This was especially the case for certain classes, like most spellcasters, that had a central class feature or features that you would fall sharply behind in if you weren't constantly progressing in that class.
Suffice to say, when it came time to redesign the system for the Pathfinder Playtest, we knew that multiclassing needed work.
Then came the rules for archetypes. The new design for this emblematic part of the game allows archetypes to be taken by any class, so you can decide exactly how much you want to invest into an alternative path for your character. The more we worked on that system, the more it began to sound like it shared almost exactly the same goals as multiclassing. Our thought was, shouldn't they just be the same system?
Multiclass archetypes are one of the more experimental parts of the Pathfinder Playtest. So much so that there are only four of them in the book, one for cleric, one for fighter, one for rogue, and one for wizard. Just like ordinary archetypes, you must take a special dedication feat to gain access to the archetype, but you cannot be of the same class as the archetype (so you can't take the rogue dedication feat if you are already a rogue). Let's take a look at one of these feats.