shared email address within family account

I'm trying to figure out whether it's possible for two members of a mailbox.org Family account to use the same email address.

We would each have our own separate mailbox.org account, but we'd also like to have shared email addresses (aliasses). Ideally:

• incoming emails to the shared address would go into a shared folder that we can both access;
• we could both reply to those emails using the shared address as the From address;
• we could both send new emails using that same shared address.

Can this be done with a shared @mailbox.org alias?

And if not, would using our own custom domain make a difference? For example, could both Family members use shared @ourdomain.nl as the From address while still keeping their own separate mailbox.org accounts?

I found documentation about shared email folders, but I'm not clear on whether sharing the folder also allows both users to send from the same alias.

Does anyone have experience with this setup?