OK, the very best but very expensive method would be to buy another power amp and use a splitter cable to feed each power amp from the pre amp.
If you want to connect two speakers to one speaker output then you could use these http

/www.amazon.co.uk/Banana-Plated-Stackable-Speaker-Connector/dp/B00G5JQ9IC]http

/.
However you should ensure that the impedance of both the speakers in parallel ( two 8 ohm speakers in parallel give a 4 ohm impedance) is higher than the minimum impedance that the amp will work at.