He is correct you should really connect the other voice coil. looks like you have dual 4 ohm coils, so id suggest running them at 4 ohm bridges on your amp for the time being.
you need to wire a series/ paralell connection to get to 4 ohms. thats running both coils on each sub in series, then both subs in parallel at the amp. heres a pic explaining a bit better.
Your amp should also indicate which terminals to use at 4 ohm bridged
http://a248.e.akamai.net/pix.crutchfield.com/ca/learningcenter/car/subwoofer_wiring/2DVC_4-ohm_2ch-br.jpg