One account can be logged in multiple times

1st doesn't work because you can't log in the same character twice.

2nd, I'm told, doesn't work because opening storage on one char locks it so you can't open it on another char. The second client freezes up (probably "hung", waiting to acquire database lock).

Oh man.. smart Nexon.

There should be plenty of possibilities, you might want to try the second method on Fredrick (Merchant storage NPC).
Opening 2 merchant with different characters, and retrieve the items together.

I don't know if it's like SEA, where the items can be picked up by different characters but same account.