I would swear that I've seen an item which allows anyone who has a linked copy to access the same extradimensional space; I'm 99% sure it was published by WotC or Paizo, and I'm 90% sure it was a belt. Similar to a Belt of Pockets or a Handy Haversack, the item connects to an extradimensional space and has the whole "items don't weigh anything or take up space" thing, but all of the attuned copies were connected to the same extradimensional space.
Basically, it lets the players hand-wave who happens to be carrying the healing potions we found back in the previous dungeon.
I want to peg the price at 10-15k per user: high for the amount of space, but with the added convenience of "yes, I do have that potion of Cure Serious".
I've looked in the Magic Item Compendium and the PFSRD, but have come up empty. I'm also striking out on Google.
Assuming I'm not crazy, where does this item live?