Phil
I have tandems in my main service panel. However, the panel is code approved and rated for them according to all of the documentation on the door of the panel.
The panel manufacturer has specific instruction for how many, and where in the panel they can be placed. There are also specific tandem breakers, id'd by part #, allowed in specific slots.
I'm assuming that the issue with precluding the indiscriminate use is the shared neutral return. Just an assumption though. ???
"The first thing you need to know, will likely be the last thing you learn." (Unknown)