How can 2 apple ID's share contacts?

You can’t, at least not directly.

You can Airdrop contacts manually between devices, or you can use a 3rd party e-mail account to store the contacts, and set it up on both devices. Something like google contacts, or hotmail etc...

Export them as vCards and pass them to the other user.

There is no method in Contacts to share data short of subscribing to the same list.

In Internet accounts, you can add a secondary iCloud account where you would be able to view the info in the other account. The types of info accessible is limited, but contacts are one of the allowed items.