This is basically a portable and resourceless sending/teleport/plane shift/gate pair of items. The utility of this is massive both in terms of movement and communication.
The primary factors that increase the rating to this level are the planar transfer abilities and the ability to move creatures massive distances. The closest spell that can do this is Gate, which is a 9th level spell and would be a Legendary level scroll that can only be used once. By making this at-will, the item has Legendary status.
Not all Legendary items are combat focused
In fact, there is a Legendary item, the Cubic Gate, that functions very similarly to what this paired/shared item can do:
You can use an action to press one side of the cube to cast the gate spell with it, opening a portal to the plane keyed to that side. Alternatively, if you use an action to press one side twice, you can cast the plane shift spell (save DC 17) with the cube and transport the targets to the plane keyed to that side.
Do note that this Legendary item has limited uses. Although it does allow the actual plane shift spell, which could be considered combat, that isn't it's only function or ability.
There are other Legendary items that are also Utility focused: Sovereign Glue and it's compatriot, Universal Solvent.
The release of this Unearthed Arcana brings another item into review. However, it's important to note that this is still Unearthed Arcana and has not yet passed balanced review in general. But the item is different enough from the proposed similarity for the following reasons:
Storage Capacity The UA item has an undefined capacity and only states that they all must have the same. It is unclear if they considered "pouch" to be of equal size as a haversack, bag of holding, or portable hole - but my hunch is that they consider pouch to be much smaller. They could be referencing a standard pouch, but it's unclear.
A cloth or leather pouch can hold 1/5 cubic foot/ 6 pounds of gear - or up to 20 sling bullets or 50 blowgun needles, among other things. A compartmentalized pouch for holding spell components is called a component pouch.
Range limitations. Limiting the range to 100 miles of another pouch and not allowing planar transfer is a major nerf compared to that of the proposed item. This along signifies a major power increase/decrease.
These two factors are significant nerfs to the proposed item and actually support my rating of the proposed item as Legendary.