No spacing except button borders:
infoPan = new Window("palette","About",[0,0,100,100]);
infoPan.button1 = infoPan.add("button", [-2,-2,102,52], "button_1");
infoPan.button2= infoPan.add("button", [-2,50,102,102], "button_2");
infoPan.show();
infoPan.center();