Full patch notes are here

[Bixby]

Additional retainers can be purchased via Optional Services.

Players may purchase up to two additional retainers. The purchase of an additional retainer will apply to all characters on a single account. However, a single retainer cannot be shared between characters on a single service account.

If I read this correctly, doesn't this mean shared storage between multiple characters

You're reading it wrong. Bolded the relevant portion. It just means you don't have to buy a 3rd retainer on each individual character. Charcter 1's 3rd retainer is still separate from Character 2's 3rd retainer, just like their first 2 retainers are.

[Rhas]

You're reading it wrong. Bolded the relevant portion. It just means you don't have to buy a 3rd retainer on each individual character. Charcter 1's 3rd retainer is still separate from Character 2's 3rd retainer, just like their first 2 retainers are.

Which can also be read (since they have the option) of multiple character accounts and single character accounts per server.

It's still good to dream ;P