My doubling rig is pretty simple. Everything I need add the DB to my slab rig fits in the DB's bag. I have a Morley A-B footswitch, a K&K preamp and a couple of extra 1/4" cables.
I just put the K&K in front of the A-B box and use it's output to match the slab's output. The slab goes into A, the DB-> K&K goes into B and the switch is as simple as stepping on a stomp box.
It works well for me. I use an Eden amp and Eden cabs and DI to the house. Everything runs flat that I can get away with it. If you are a tweaker, it may not be ideal.
These days, I have given up on worrying about my sound. The Eden sounds really nice flat, and the DB just fine with the revolution and nothing else for pizz.
If I also have to bow, I just ask the sound guy to set a mic out for the arco stuff. Funny how I
always seem to forget to switch the A/B over to the DB's PU when doing the arco stuff.
My bass is pretty loud when bowed. I don't need much anyway.
I was using a Raven Labs USIP, which sounded absolutely amazing as a blender pre for the DB or my acoustic guitar, but didn't work well at all as a two-instrument setup. The tuner out fed from only one of the channels, you had to run two DIs to get a decent sound and a single side of the preamp didn't sound that great for slab anyway.