As long as the woofers are IDENTICAL (ie, same model) wired in PARALLEL, they should work fine. If they're in SERIES, they should optimally be in seperate compartments. Parallel woofers can "self-balance" themselves pretty well...

At less-than-300 Hz, a pair of woofers should pretty much act as one big woofer. Wavelengths at 300 Hz are about 3 feet... so, pretty much any woofer 15" or smaller should work paired up at that crossover point.

Regards,
Gordon.