Each person creates a username and shares it with the other. Send a friend request and accept it. The messages are free. You do not need a group, and you do not need random chat.
Decide what the shared phone should see. App lock covers the whole app. A hidden chat covers this thread. A chat PIN covers the messages if the row can stay visible.
Photos and distance
Photos you both want to keep can live in the vault and be shared with each other. A share is explicit, and it ends if you unfriend or block. Photos that should not linger in the thread can be view-once or on a timer.
Long-distance pairs can mute during work hours and use quiet hours overnight. The thread stays. The phone stays quiet.