As a DM, I find the correct answer is to just grin and get them to make an arcana roll. If they make it, they are aware that other mages must have thought about stacking dimensional pockets, but nobody does it. When the mage's mentor was asked about it, he just said "heh, well...I wouldn't".
That is enough fair warning. They can try it if they want and if it is non-abusive, nothing happens. Now however, I have a good basis for whatever nastiness might happen if they abuse dimensional spaces. In reality though, they just know to not mess around doing stuff like that.
For me the more common issue is putting living beings and people inside bags of holding or dimensional pockets. I just handle it in the same way.
That is enough fair warning. They can try it if they want and if it is non-abusive, nothing happens. Now however, I have a good basis for whatever nastiness might happen if they abuse dimensional spaces. In reality though, they just know to not mess around doing stuff like that.
For me the more common issue is putting living beings and people inside bags of holding or dimensional pockets. I just handle it in the same way.