I think the 2 slots to be left open are to install a double pole breaker that will work as a shutoff for that subpanel. Flip that one and kill the whole box.
Here's the Square D catalog info on QO breakers. Says that tandem breakers can only be installed in certain positions on the panel.
Best to understand exactly how mfgr states they can be used
From what I've heard, the smaller breakers have less room to dissapate heat and can therefore fail more easily. If the box isn't sized for the extra circuits, you can exceed fill capacity, run out of neutrals, etc.
Seems that they are a way to squeeze just one more circuit into the older installations so you don't have to upgrade the entire service.