I understand the pros and cons of multiclassing early, but I don't understand it regarding a character that has reached level 20 in one class and decides to continue playing their character with a different class.
There is also the question as to if you need to select a different class or can you go into it again? EX: Level 20 Sorcerer wants to go into Sorcerer again.
Would the caster level increase to 21 or would it just stay 20?
Base Attack Bonuses increase how?
Would a Fighter's DR stack with the second Fighter's DR?
Can you gain more attacks per round? Ex: level 20 Fighter and level 20 Gunslinger would be? +40/+30/+20/+10 or +40/+35/+30/+25/+20/+15/+10/+5 or +40/+35/+30/+25?
This does not need to be answered quickly, but an answer to each would be greatly appreciated.