I've had plenty of experience with the K&K Bass Master Pro and their two pickups since I started selling them over three years ago.
Key is to treat and EQ them separately, and play to each of their strengths and characters, and then use the internal gain controls to equalize their volumes-- the Bass Max side is very hot and doesn't need much or any boost, but the Double Big Twin side, due to its nature, needs some juice.
Personally, I blend them depending on the type of gig. I like to favor the rounder, warmer Bass Max side when I'm doing country or blues -- as well as for arco, though I'll bring a little Double Big twin in for articulation and string sounds if I want it. Fingerstyle jazz makes me lean towards the stringier, almost close-mic'd sound of the Double Big Twin side, but with some Bass Max for richness. The combination adds dimension IMHO.
To me, the blending of the two pickups is a great advantage; I've had other bassists add one of the two pickups, along with a 2 channel preamp, to enhance the coverage and improve the versatility of their other, non-K&K pickup. No matter whose pickup you have, two can be better than one.
Greg Hippen, Bassist for Garrison Keillor's "A Prairie Home Companion" radio show told he he clips his preamp to the edge of the music stand during shows so he can change characters on the fly, given the wildly eclectic nature of the material he plays for their weekly shows.
As with any pickup, it's always an advantage to get somebody else behind your bass so you can hear how it sounds from an audience perspective. I've found a less bottomy sound on stage helps clarity -- and heard far too many basses whose boomy reflection on stage turns to mud.
Shameless plug:
http://www.gollihur.com/kkbass
I have an Acoustic Image (nice little combo!), but also have a Clarus on the way that I hope to pair with the Euphonic Audio 208 cabinet I recently got. I read of its dimensions, but I guess will find it hard to believe until I have it in my hot little hands.
