1) What do you think the most likely cause of the first sub blowing was ?2) Have you ever heard what it sounds like when one sub blows in a common-chamber enclosure ?
Answers:
1) Too much power
2) No, otherwise you'd know that you'd know right away if one of the subs were blown. In this case, separate chambers would offer you the convenience of being able to use the other sub(s) while the other one was getting replaced/repaired. However, this in no way means that it offers more protection, unless you have extremely slow reactions, are completely deaf, or are operating the sound system while being away from the vehicle.
On the flip side, if you're operating a separate chamber enclosure, you may not realize that one sub is blown and, since most people bridge their amplifiers and most amplifiers output more power per sub at a higher impedance, the likelihood of blowing the other sub would increase as well.
i.e.
500 w @ 4 ohm
900 w @ 2 ohm
1500 w @ 1 ohm
Two dual 4 ohm subs wired in parallel-parallel, each receiving ~750 watts. One blows, the other sub is now receiving ~900 watts.