Now... I've just had another thought. From experience the last two double stem bubble MOC's I have owned were encased in graded casing (one AFA and one UKG) and both came away from their cardbacks during transit to myself, one almost completely.
What I am getting at here is the fact that the double stem bubble seems to not have been the greatest at sealing as there was very little litho damage when this occured. One was on a Meccano MOC and one was on a ROTJ Clipper MOC, neither would have been a TT.
So... now I am thinking is the stronger seal on a double stem bubble TT or vice versa?! Lol, I can hear a tune, 'Never ending story' comes to mind. :?