Sub woofer cable help please...

Sub cable

I recently purchased a single sub woofer cable, but I have two outputs out of my preamplifier. I am ok going into my subs single input but coming out of the preamp is an issue. I have two female RCA outs of my pre amp and I need a y adapter with two male and one female. Do you know where I might find one or any suggestions? As it stands now I can plug it in but only to the left or right output from preamplifier. And then into the one input on the sub. Thank you...