I don't believe he meant that the other enclosure would be connected but just sitting there and the answer as to what it would do would be: Nothing.
The reason why nothing would happen is because nothing is exciting the port with any real force.
I usually blow off my enclosures with a leaf blower and if you blow them off before the sub hole is cut out, you can hold the blower at just the right angle to excite the port. Even with a ton of air exciting the port, it still only lets out a very small sound that is the same frequency of the tuning frequency of the enclosure. You can also excite it after the sub hole is cut by blowing over the cutout. Either way you get the same effect: A sound with the frequency of the tuning frequency and very little output.
If the enclosure was just sitting in the trunk or where ever, it would only get excited when notes were played right at the tuning point and even then, you wouldn't hear any difference at all because of the very small amount of travel. This would be pointless to do.