My son and I play Minecraft together on XBox.
When I create a new world, or load an old world, he is able to join that world without issue.
When my son creates a new world, or loads an old world, I attempt to join and get stuck at the "building terrain" screen.

I have attempted rebooting. Power cycling. Validating internet. Checking settings. Loading the world an existing the world. And many other options I found online and in forums. Nothing improves the situation.

This is not a situation of remote access. We are both sitting in front of the XBox. Which each have a controller which is connected to the XBox. We can each play a world individually without issue on our own time. My son can join a world I created and get right in. I cannot join any world he creates because it hangs up on the "building terrain" screen for my player view on the split screen.
This happens regardless of account attempting to join a world my son created. I can create a new account and we can use that to join my worlds. We can do this also with the guest account. 
However, when he starts a world (old or new), any additional user gets caught at the "building terrain" screen.
Again this is not remote. This is not realms. This is two people sitting in front of the same TV, playing on the same XBox, side-by-side.