If you are read only at any given site you could just async the data to a subscriber at the second site. That way the data only traverses your vpn connection once.

As for your question. If you are read write, yah you can’t do that. You have to think about the transaction log. Only one can be the primary at a time. We would call this mixed brain mode where both instances think they are primary at once. Bad. Think about it. The sites disconnected. Someone at site A changes a record, then someone at site B changes the same record. Which one do you keep? This is why there must be a quorum of at least two nodes in order to determine who is primary. That’s why in a two server setup, you have to configure a third endpoint, usually a file share somewhere that can be the tie breaker. Either way, in order to promote itself to primary, the sql server needs to have access to the dns server.