Two places where i know its possible to do this and indeed try to do it everytime are. The final hill before the tunnel on sebenco forward. If you get the right position you can BR off the hill, boost down and BR again while in the air.(Leungbok posted a clip of him doing this).
Also Ubermall reverse after the little chicane, BR off the little hill then another one before you land. this is slightly faster than doing one BR (I think)