Dual voice coil speakers are used when you don't need to cover a large area, or when you do and having a discrete left and right speaker means the sound from on isn't audible from the other location, or if the delays would be a problem- ever listen to old Beatles music from the original recordings through headphones? You would have noticed that the music 'ping pongs' between the left and right channels.
Odd numbers of speakers can be a problem, even with volume controls- wiring in series means the crossover in the first will have an affect. Wiring parallel also causes problems- If you can add speaker wires to the head end from the first volume control's location, or if you can add volume controls at the current VC location, use the kind with jumpers for impedance compensation- that's the only way to do this without adding resistors and creating heat, which could cause something to burn.