It looks like a bandpass using both woofers would need a 1 cu. ft. rear chamber, and a .625 cu. ft. front chamber. The front chamber would need a port 3" round by 6.4" long. This would be the ideal box, but you could get away with a .75 cu ft rear chamber fully stuffed. This set up should give you ~5-6db more output from 50-85 hz, than your infinite baffle setup. It rolls off pretty sharply under 50 hz. With cabin gain, you should have usable response into the mid to low 30 hz range.
