Our setup:

2 different xboxes 

2 different xbox live accounts

2 purchases of Minecraft Dungeons

1 household (we connect through the same modem)

Our issue: 

While playing "online" so that we can play together from our different rooms we crashed 3 times without being able to complete a level.

While both xboxes were connected and playing offline, we also crashed just a couple minutes after our level launched. 

We experience no game crashes so long as only 1 of our xboxes is connected to Minecraft Dungeons.