You may want to consider another option. There are some really nice half and third rack pres out there. If you get a stereo pre, then you can dedicate one channel to each pickup. If the pres have inserts, that would also allow you to insert compression or related dynamics into the signal chain. If your bass amp has only one channel in, then use a "y" cable to drop the two stereo channels into the bassamp. You'd need to take some care in the gain staging, but it shouldn't be all that tough to balance it.
One solution that comes to mind are 1/3 rack units made by FMR audio. There are others, too; Summit makes a mono 1/2 rack, but you'd need to buy them in pairs. As for full rack mounts, a pair of Chandler Germanium pre/DI units would be killer in the application you describe. So would a pair of Avalon U-5s.
As for the FMRs (no this is not a commercial) they offer a unique fit as a system for the use you're inquiring about. The stereo preamp has inserts <http://www.fmraudio.com/RNP8380.htm> for their two compressors they offer; the RNC is a really clean and transparent compressor <http://www.fmraudio.com/RNC1773.HTM> and the RNLA is a darker and more colored sounding leveling amplifier <http://www.fmraudio.com/RNLA7239.htm>. This would offer a robust set of sonic options for your bass pickup feeds.
I've used the RNC and RNLA in my studio work and in some live situations. I've not used their stereo pre, but the FMR folks do first class work. It just seemed a natural for the question you asked, whether or not you're interested in adding compression, outboard EQ or other dynamics to your current bass rig.
