Isobaric lost favor in the car audio market right around 1994 or 1995 when the solobaric was designed. It was the first sub to add mass to the cone (which simulated an isobaric design) to lower the Fs and allow a much smaller box than was tradionally seen. The problem was that it had the same effect as an isobaric design in that it lost a substantial amount of effeciency. It was a marketing strategy that payed off, because companies like JL, MTX, etc soon followed with similar designs all with lower effeciency.
Obviously the biggest advantage to the design is that 2 speakers will fit into a box 1/2 the size of a single woofer. If space is at a premium, it's still a viable alternative, you just need to realize that you will need 2x the power and 2x the woofers to get the same output as a single woofer.
Wjang is correct in some cases. It depends greatly on the design. The application that uses two woofers to reduce second order harmonic distortion most effectively is a sealed subwoofer design like M & K push pull home theater subs that aren't isobaric but do have the cone pointing out on one subwoofer and the magnet pointing out on the other.
Aaron, that wasn't an isobaric design, that was an infinite baffle.
