You will need to put both on the main couch and be sure to mount them so that they affect the entire couch and not just one or two seats. Might need to add a support underneath to do that. The BKs obviously will do the job but are as you pointed out twice as much for one versus two. I bought two and put one on my recliner and one on my recliner couch and wound up buying two more to get the couch evenly and completely covered.
Auras work, and on a tight budget get the job done!
// Front Projection Enthusiast //
BenQ W1070 - 103" EasyFlex Painted Screen
Onkyo TX-NR646 - JBL Northridge 5.1 - SVS PB-1000
Roku 3 - Amazon FireTv - Sony BDPS5500 - Kangaroo PC