I am currently running those same two SansAmp preamps in parallel. Here is how it is done, courtesy of TalkBasser JGR: Something cool for Sansamp RBI/RPM users...
It sounds fantastic. What I do is set the RPM for clean lows/low mids, and the VTRM for dirty mids and highs. When run in parallel you pretty much get the benefits of a stereo bass rig while using a mono amp/speaker setup... not exactly the same, but close enough IMHO.
I used to run RBI and RPM in parallel as JGR described, although I preferred to use the RBI as primary and RPM secondary. Another TalkBasser runs RBI, RPM, and VTRM in parallel! I'm sure that works great too, but I want to keep my rack at four spaces (two 1U preamps and one 2U power amp).
I'm not sure how well the parallel system will work for other brands of preamps (there might be impedance/output level issues, and not all preamps have an FX loop), but it works beautifully with the SansAmps.