Quite easy. Just make your new entites using entity editor. The portrait filename to put in core/gui/char must be the exact same name of the char name added in entity editor. Save those entities in core/entities/characters
After that, just open the mission, select the tab "entity edit", select Stich and press the "..." button right of the entity file path (top bar). Then select one of the two entities you made. Do the same with Farsight and thats it!