Account Already Logged In Error Fix

Though I just tried, and using storage on one, and trying to access it on the second makes the 2nd account unresponsive till you relog.

This maybe be why they're not announcing it as a great new feature. They may be unsatisfied with this form of duping-prevention. To an innocent player, having storage hang your game just because you previously used it on another char, seems like a bug. Not to mention that being blocked from using Cash Shop on all but one char must be very distasteful to Nexon.
Annoucing "You can now log on two different characters of the same account at the same time, but may only use cash shop and storage on one of them until you relog" doesn't sound very good either.
So they may not want to announce this until they have a more elegant way to prevent duping.

Another possibility is that they are "beta testing" this feature, unannounced. They knew the community would discover it, and that some people would immediately set to work trying to find ways to abuse it. They are keeping a very close eye on the database and packets of anyone double-logged, watching for any sign of illicit gains. They will only announce the feature to the general public once they have reassured themselves it can't be abused (beyond the obvious self-leech, etc).

The third, and most likely (imho), possibility, is that this is a glitch. They weakened the duplicate-login detection so much it no longer works at all. The block on storage and cash shop are coincidence, a side-effect of the way the database works: it locks records or entire tables to the client that first accessed/modified them, until it exits.
The strongest evidence to support this possibility is the fact that if you try to log in the same character twice, the client just crashes, instead of giving a "this character is already logged in" message.
That, and what we already know of Nexon. The first possibility would require them to care about players getting stuck and having to restart the client; the second possibility would require them to dedicate a lot of attention to exploit-prevention; the third just requires them to be incompetent and somewhat lucky.

It was fixed anyway.

Was not.
Tested just now.