... If you don't even know what I'm talking about there, how did you even make your own client in the first place >_<
It doesn't get more simple than that ):
You get a .DIF File, you get a DIF Editor, you get an untouched version of Sakray, and you open it up with the DIF Editor and select stuff you want; one of the options is "Multiple Windows" or something similar to that. Permit that, patch the DIF, and tah-dah. All those files can be found at the eAthena board.