Sharing a mailbox between different tenants

Hi all, very new to Exchange so appreciate what I'm asking might not be possible, or just plain dumb!

The problem: I work for a small company that's recently been acquired by a large firm. We're both running O365 etc but obvs as separate tenants. Long-term I expect us to consolidate this but in the short term we're BAU. We have a shared mailbox for our HR function that members of the HR team at our new parent company now need to access.

I've accepted that the most straightforward way to do this is to just create user accounts for them on our tenant and add them to the shared mailbox that way, but I can't seem to do that without assigning them an Exchange Online license first. They obviously already have these assigned to them via their parent company's tenancy.

Is there a workaround for this? I don't want to have to pay for more licences just for a temporary solution.