When Pillars of Eternity 2: Deadfire first begun its Fig crowndfunding campaign Obsidian put multi-classing as the initial stretch goal. This naturally led to some concerns about balance, because trying to keep a couple of classes relatively equal is a nearly impossible task, let alone trying to do the same balancing act with fifty of them!
In order to put some of these concerns to rest Obsidian has recently posted a detailed video going over how exactly multi-classing will work in Pillars of Eternity 2: Deadfire. If this sounds incredibly boring to you, worry not as Game Director Josh Sawyer has quite the talent for making explanations interesting. Anyway, here's the video:
One of the biggest issues with multi-classing systems is the simple fact that most combinations end up being either useless, or downright overpowered. As such, I am incredibly glad to hear that Obsidian is introducing an array of secondary stats to hopefully make all playstyles equally powerful, while still giving a slight edge to 'pure' classes. The classes most likely won't be balanced, and I'm sure there's going to be an optimal path for min-maxers to take, but the important thing is that due to this new system every single permutation will have a fighting chance. And for a singleplayer game that's pretty much all you can ask for, because it doesn't matter what class is the strongest, it only matters that the one you find fun is strong enough to complete the game.
If you would like to learn a little bit more about Pillars of Eternity 2 and its lore head on over to the Fig Update page - there's a few new bits of information there.