The infused pouches all share one interdimensional space of the same capacity as a single pouch. Thus, reaching into any of the pouches allows access to the same storage space. A pouch operates as long as it is within 100 miles of another one of the pouches; the pouch is otherwise empty and won’t accept any contents.
At first glance, it seems like the clause in bold limits the magic of the pouches to a range of 100 miles. However, things are not quite so simple. If I were to infuse 4 pouches and then keep a pair for myself while giving the other pair to someone else, then every pouch will always be within 100 miles of another pouch, which seems to imply that every pouch will continue to function. (Other similar variations are possible as well, but this example demonstrates the principle.) Even if the other person were to travel to another plane, nothing in the wording seems to indicate that the pouches will stop working.
Hence, at any time, one of us can write a note and put it in the pouch, where the other one can find and read it. Compared to spells, this seems to be roughly on par with the 5th-level spell Dream when used for the purposes of communication. (For example, Dream doesn't work across planes, but can target anyone rather than just the holder of the other pair of pouches.) And that's before considering the non-communication uses of the infusion. A more direct comparison would be a pair of sending stones with unlimited uses.
When read in this way, it seems quite a bit more powerful than what should normally be available to a 4th-level character. It also seems like a non-standard use of the infusion to keep the pouches in pairs rather than handing out one pouch per person.
Is this an allowed use of this infusion as written, or have I missed something that would prevent it from working in this way?