The attached photo shows the error message.

Issue relates to trying to login to a Bedrock realm (playing on a Switch).

I have:
1 switch.
2 separate user profiles both subscribed to separate Nintendo Online Accounts.
2 separate Xbox/Microsoft online user accounts - both are successfully invited to the Realm.
Both of the above Microsoft accounts are logged into Minecraft under one of the two unique Nintendo online accounts.
Each account is able to successfully join the realm and play.

However:
When either of the accounts is logged into the Realm and either of the other accounts also attempts to join, the initial account is kicked out and the following error message is displayed:

"Cannot join world.  The account you are signed into is currently playing in this world on a different device."  The account attempting to join sticks on "Building Terrain".
Picture attached for reference.

Note:  this is a recent issue and both accounts have previously been able to login and join the Realm, playing on split screen.

Attempted fixes:
Logged out and in again on all Nintendo and Microsoft accounts.
Reset the Bedrock Realm.
Fully restarted the switch.
Checked that the latest version of Minecraft is in use on the Switch.

The only possible issue i can think of is that both of these accounts are Child accounts with the overall Parent account being my personal Microsoft account.  I play on a PC and have zero issues.