main class and dual class

hello guys i got a question to ask.whats the difference between main class and dual class?is it only the oly participation or there are other things aswell that you cant do with dual.
because i was thinking on making archer main and healer dual but i would like my archer to be trickster cause of the pvp dmg which i believe makes trickster dmg far superior than others(correct me if im wrong),so i thought to make healer as main and then sub a trickster and make it dual class.

thanks in advance

p.s. additionally is there a problem if i want to leave my main class 85-90 and get my dual to 99?