First off, fighters roll a ten sided die for HP. Clerics roll an eight sided die. Barbarians, twelve sided. Thieves, six sided. Mages, four sided.
If I am a first level fighter/first level mage. I take a ten sided die plus a four sided die and divide that by two. Then add the constitution bonus. I haven't actually watched the game rolls on multiclass, but that is the way it works.
You don't get the constitution bonus twice. (Fighters types are the only ones that get any bonus above 16.) When you level up, you get the class you level up with, divided by 2. Ie..Fighter..roll a ten, you get 5. Mage roll a four, you get 2.
