using a common chamber enclosure

I can see a disadvantage only if each of two subs in a common sealed airspace are being fed separate stereo signals, which may cause a greater degree of cancellation.  Although most recordings in the sub range are mono, the possibility exists that a source may have different signals in that frequency range carried through the left and right channels.  Subs are typically wired together and fed a combined signal, which eliminates that potential problem.  Other than that, I see airspace as what it is...airspace.  The subs should function the same either way.

Vented subs, on the contrary, are more prone to damage if one of the subs should fail.  The quantity of airspace is more critical in a vented application than it is in a sealed.  Most of us (I think) recommend separate chambers in vented enclosures.

As for mutual coupling of subs, just say no.  There's too much to worry about in this day and age to have to also worry about what's going on in the trunk of your car.

why would a common chamber look different than a box with seperate chambers?  I can't rap my mind around any type of box design that if it had one chamber, would look different if you seperated the subs.

Also, I disagree with the notion that most people focus on what happens when the subs blows.  You're last problem in that scenario, is if the box is the wrong size.  You should be focusing on the fact that you did something wrong in the install that would cause your sub to blow in the first place.  Keep the focus on what the subs do in a box while they are healthy, not when they die.