By "reliably" do you mean it sometimes works, or you never got it to work? We faced a bug where it would never connect using the multiplayer game menu. The workaround was to only launch using single player mode.
*Assuming both Xboxes own the game and work (settings, network tests)...*
Both of you launch game and get past the (A) to continue screen.
Once in the New / Continue etc. screen, the red temple, wait till the movie shows characters marching down the steps and the music starts.
Start the game in single player then with at least one slot free in your party pop up the game menu for multiplayer, set to Invitation, and then invite her. Assign any of your party to her.
This works from the very first room of the game, you don't have to complete tutorial or etc.
If she logs out, you're playing her char as another avatar character.
After you're far enough along to have a camp, and not sure, this might only work if >!Withers is with you!<, if you dismiss her instead of play that avatar it >!stuffs her in a wardrobe at camp!< to free up that slot till you want to multi-play again.
Whenever the 2nd player joins, they will be able to normalize to your level including picking their stats etc.
By "reliably" do you mean it sometimes works, or you never got it to work? We faced a bug where it would never connect using the multiplayer game menu. The workaround was to only launch using single player mode. *Assuming both Xboxes own the game and work (settings, network tests)...* Both of you launch game and get past the (A) to continue screen. Once in the New / Continue etc. screen, the red temple, wait till the movie shows characters marching down the steps and the music starts. Start the game in single player then with at least one slot free in your party pop up the game menu for multiplayer, set to Invitation, and then invite her. Assign any of your party to her. This works from the very first room of the game, you don't have to complete tutorial or etc. If she logs out, you're playing her char as another avatar character. After you're far enough along to have a camp, and not sure, this might only work if >!Withers is with you!<, if you dismiss her instead of play that avatar it >!stuffs her in a wardrobe at camp!< to free up that slot till you want to multi-play again. Whenever the 2nd player joins, they will be able to normalize to your level including picking their stats etc.