Class combo with other Races

i wanna know how to get some interacial combo lol no seriously i wanted to try tanking as Knight/Warden and saw some post saying that it was possible over here in arcadia to do so, so i was wondering where and how to get this done. asked some guildies said they tried but the class trainer wouldnt let them learn the class that's not his racial class

says i dont have enough diamonds but i do

yeah i did 2 time now

Edit: worked the 3rd time ty though