I had been running a Lulz dual extruder for quite a long time, had no issues with the flex plate warping.
I did, however, have issues with the design as a whole (that thing is HEAVY and puts a lot of wobble everywhere!) so I ended up moving to an E3D Chimera. For a short while I was running it as a bowden, but now my fancy-shmancy Servo extruder has arrived, and it looks like this:
Getting it running smoothly was… not easy. Lots of firmware hacking, running some extra cables, a step down voltage convertor, etc etc. But, I can run two extruders with a single stepper motor! The servo acts kind of like a car transmission, shifting the motor between two gears. The mount I designed is also rigid as hell, and since everything is very, very close to the smooth rods there’s not much flex (if any).
The extruder is the HPX2-MAX by Dglass3d. For the moment, I can’t really recommend it unless you like a challenge - there’s almost no documentation on it at all yet, and the modified firmware they use is either much older, or much newer than the TAZ firmware - I had to tweak a LOT of stuff to get it running nicely. The Dglass guys are relatively good with information, but all my communications with them suffered a lot of delays. Basically, very nice product, but they need a lot of work on the support side.