Shameless plug for something I currently have for sale but if you need to run two subs and your preamp only has output for one, then a JL Audio CR-1 crossover (along with a few others) is what you need. If you don’t have your subs already, I do highly recommend JL Audio subs. I have had and still own 2 Rel subs and an M&K sub but the JL Audio subs are better (F112v2). With the crossover, you don't have to limit your preamp search to only those with two subwoofer outputs, any preamp with one subwoofer output will work, which is the majority of them out there.