I have the same problem and I fix it with the following :
//can open left and right panel
menu.setMode(SlidingMenu.LEFT_RIGHT);
//can only open left panel and show only left (perhaps overlay the right panel ?)
menu.setMode(SlidingMenu.LEFT);
but it only works to "disable/make invisible" the right panel if you do :
menu.setMode(SlidingMenu.RIGHT);
it only show the left panel on the right.