If only one set of outputs at a time are valid with your preamp you could change your preamp, or get a Y-Cable that plugs into your preamp or get a custom Y cable (my friend had this option done so he could biamp with 2 amplifiers) if you using RCA's Monster makes a single solid 1 into 2 adpaters (audio advisor sells them) I use to use these so I could run my single sub output to two subs.
I would not try running both off your amp.. only bad things could happen..
I would not try running both off your amp.. only bad things could happen..