before being able to use both classes?

it's a big mystery...

In AD&D rules, I think the justification was that to change from say a fighter to a mage, a person would have to stop practicing their fighting skills, and apprentice themselves a mage. Since in BG2, you are gaining levels as a mage not from learning from another, higher level mage, but because your party is killing stuff, it doesn't make as much sense.

I think the real reason behind the rule in the first place was to impose some kind of penalty on dual-class characters. If they never lost their original class abilities, then there would be no drawbacks at all to dual-classing.

The 3rd edition rules about multi-classing have no such penalties, but they allow all races to multiclass.

Interesting.

Of course, the most realistic system is to give EVERYBODY basically the exact same abilities, and then, the only way to become more skilled at those abilities is to practice them. So, for instance, you're a Fighter/Cleric/Thief/Mage, and you spend your day whackin' bad guys with your axe. Bang, you've gained a level as a Fighter and your skill with Axes has increased. But if you had spent that same day sneaking around, breaking into buildings and stealing things, your Thief level would increase and your Lock Picking and Stealth scores would go up.

Will SK allow you to give thieving abilities to non-thieves?

Correction: multi-classing in third edition is not completely without penalties; if neither of the two classes is your race's "favoured" class and one of them is a lot higher in levels, you'll get a 20% XP penalty. But not as bad as the 2nd edition penalties, no.

Yes, but the game wont allow you to use them. I dont think they will even display in the character record, probably only in the level up screen.