Remember that when you connect speakers together, the impedance changes. When you connect speakers in series, the impedances add. (An 8-ohm speaker connected to an 8-ohm speaker results in a total impedance of 16 ohms.) However, connecting speakers in parallel divides the impedance. (For example, two 8-ohm speakers connected in parallel result in a 4-ohm overall impedance.)
Make sure the total impedance of your speakers is safe for your amplifier.