help on surround speaker placement

[reyz]

hi, need some help on surround speaker placement!

currently have a 5.1 setup and my viewing room measures 14' x 21', breadth x length. however, my viewing room is essentially split up into 2 sections:

the living room and the dining room.

basically the comfy couch seperates the 2 at the 14 ft mark from the front wall (tv and front speakers), although technically it is the same room in all. the back portion has the dining table, a glass cabinet and a couple doors on both L and R side.

what i need to know is whether i should place the L and R surround speakers along the sidewalls just behind my couch position, angled towards the sitting position OR place them at the far wall right behind.

problem is if i place it at the far wall back, the distance from ear to them is double that of the distance to the front speakers. the sidewalls are a better more accurate distance option but the thing is that there will be then an empty space at the behind (aka the dining room). will the empty space behind take away the surround sound effect???