If you are going to be connecting 2 bookshelf models together, they need to be place vertically, otherwise you are kind of missing the point of getting a bookshelf.
There have been some studies suggesting that a speaker with a vertical placement has a wider horizontal dispersion than a horizontally configured one.
You have 2 options on how to connect them:
Series and Parallel (image courtesy of pmbuko)
If you wire them in Parallel, you will gain a 3db output from your speakers but your impedance will drop by half. So if you are running 2 8ohm speakers, your new impedance will be 4ohm (your receiver shouldn't have a problem with this).
Parallel is the perfered method of running 2 speakers on the same channel.
A series connection will decrease your impedance by double, so running 2 8ohm speakers will give you a 16ohm impedance (really easy to run), but your db level output goes down (not sure by how much)
Edit: I should also add that the reason I wanted 2 speakers in parallel (I normally would have purchased 1 speaker) is because I am going to be putting one above and below my 92" projection screen, to make it sound like dialog is coming from the centre of the screen.
