Can you only create one Hardcore Character at a time?
If you can create more than one Hardcore Character, can the Hardcore Characters share the Items in the stash?
You can create more than 1.
Yes, they can share things in the stash. This is typical HC behavior. You may get a full set of gear for your build, but many HC will keep 1 or even 2 more sets for backup.
The only Hardcore Characters that cannot share are Hardcore Seasonal Characters and Hardcore Solo Seasonal Characters.
However, when the Season ends, Hardcore Seasonal and Hardcore Solo Seasonal items and Achievements are combined and sent to your Non-Season Hardcore Game Mode.
The same is true for Seasonal Characters and Solo Seasonal Characters.
So, if you want your Hardcore Characters to share everything during the Season, they must all be Hardcore Seasonal or Hardcore Solo Seasonal.
They also share paragon, of course if we talk about same gamemode.
That is cool. At least they are not 100% standalone.