One additional wire into a two-channel Clarus, B String. No mixer required. Sorry, my computer won't do quotes for some strange reason.
I have a Focus I. Its one of my all time favorite amps. Very user friendly. For a number of years I used it with my upright into the 10 megohm input and my electric bass through a Tech 21 Paradriver, into the effects loop return. However...
When I was blending two pickups on my upright bass I needed a phase switch sometimes and better eq for the two pickups. The RavenLabs PMB blender is a really great piece of gear. Too bad nobody's making it now. That works great.
I've thought about getting a two channel Acoustic Image head and using one of Fdeck's preamps with the phase switch and 10 megohm buffer for one of the pickups. (Piezo pickup) Again, after all that "stuff", its nice to find a pickup that just "works" on my bass.
I remember seeing Brian Bromberg's bass with a slew of pickups on it all over the place. He went in the studio to record and only used one DPA mic.