If I have Emp at Arcane Degree, have I +20 to Attack or +40?

you would have +40, each levels attack bonus is separate, though do keep in mind that these do count towards the class bonus maximum of +50.

a good example of how this works is to look at Lama; it grants +10 to unarmed defence at base degree, no bonus at advanced degree, and +10 to unarmed defence at supreme degree. and it would just sound weird if someone lost the ability to defend for a degree just to gain it back.

but i just now noticed that emp's +40 is pretty wild, that's most of the class bonus maximum right there, ouch. so you may not have a bonus at all if you give that art to a combat class.

The bonuses of Advanced Martial Arts can surpass the limit of +50 BUT you can't add together bonuses from different Advanced Martial Arts, you apply only the highest.

oh, i didn't know that, looks like we helped each other out.

Actually, you can add the bonuses from martial arts together. The only things you can't add togetther from martial arts are the base damages, and the master bonuses, which are different from the normal bonuses. You can add regular martial arts bonuses together, however they count towards the maximum +50 limit for Attack and Defense. While you can't add master bonuses together, you can use more than one, say you had +20 master bonus from Arcane Emp, and +20 Block and Dodge from Arcane Selene, you could use both of them.

:edit: double post, cursed internets