Cancellation shouldn’t be an issue unless you are in fact running a "stereo low frequency signal" through them with differing signals on each channel. However, most bass / low frequencies in music is mono, so cancellation by running a pair of woofers, with a proper low pass filter, should not be a problem at all. I'd be more concerned with mounting them well and sealing the baffle. Of course you could run into a song with oddly recorded bass, which could perhaps cause an issue, but rather unlikely. If you used a mono amp, or a stereo amp bridged, this would stop that rare possibility. People have run subs mounted like you propose for years. Google "infinite baffle install" and or "free air subwoofer install" and you will find photos / forum threads on the subject, do some reading on it as you decide / plan.
