for reinforcement of bass you can put sub facing corners. placing subs is less of a concern if you adjust bass crossover to bellow 60hz. if you need to adjust 80hz or above, you'd be better of placing them both next to speakers.
Sub placement
Many speaker manufactures recomend placing the sub in your listening spot,then go around the room and find where it sounds the best. Where it sounds the best is where they say to put the sub. I have 2 foward firing subs. Does any one have any idea what direction I would place the sub when I find the spot? Has anyone had any luck with this method?