You can easily have two seats on the same flight. People transporting a double bass do it all the time.

Whether you can do what you're suggesting (the second seat being the result of a second booking ultimately to a different destination) depends on whether the computer systems flag this as a double booking (which may lead to a cancellation) or not. Your chances are probably improved if you didn't include your FF number on one of the bookings.

My question though is whether this is significantly cheaper or not. I'd be surprised if it was.