[SixOfSpades]

Radek, your summary is mostly correct. Here's the full list of requirements:
1) You must be Human.
2) You must be at least Level 2.
3) You must have a score of at least 15 in the primary stat of your current (1st) class. (STR for Warriors, WIS for Priests, DEX for Rogues, INT for Wizards.)
4) Your 1st class and the class to which you want to Dual must form a valid multiclass combination. These are: Ranger/Cleric, Fighter/Druid, Fighter/Thief, Fighter/Mage, Cleric/Thief, Cleric/Mage, and Thief/Mage.
5) You must have a score of at least 17 in the primary stat of your 2nd class.
6) You may only Dual to a class that does not conflict with your Alignment. (Lawful Good characters cannot Dual to Thief, characters who are not True Neutral cannot Dual to Druid, and Neutral and Evil characters cannot Dual to Ranger.)
7) You may choose a kit for your 1st class, but NOT your 2nd class. (Which I, personally, disagree with. That's like saying a brain surgeon can become an auto mechanic, but a doctor can't become a German auto mechanic.)

There are certain exceptions to the rules listed above:
a) To Dual to Druid, not only must you have 17 WIS, but you need 17 CHA as well.
b) To Dual to Ranger, you must have the required 17 STR, but also 17 DEX and 17 WIS.
c) Archers and Stalkers (Ranger kits) cannot Dual. (For some bizarre reason that I think is completely retarded, and shouldn't exist at all.)