Dual Class - Experience; Class Switching, Advancement

[CaptainHarlock]

I have a question regarding dual classes and experience.

Suppose I achieve level 170 in the first class. I then option to create a dual class for a second class.

I was wondering for that second class to achieve a level does it take as much experience as the first class listed to go from level 170 to 171 or does it take as much experience as a level 1 class to a level 2 class.

Suppose I go away from my computer and place my ship in a defense mode for the clan's fort. Also suppose that I am still playing with my dual class ship with level 170 in the first class and level 1 in the second class. When I place my ship in defense mode for the clan's fort can I switch my ship to the first class and then later place my ship back to the second class when I return to the computer and play the game?

I was also wondering when you choose to play dual class are you able to still advance the first class that you selected?