Operations under which the class of undecidable languages isn't closed

Do there exist undecidable languages such that their union/intersection/concatenated language is decidable? What is the physical interpretation of such example because in general, undecidable languages are not closed under these operations?

What can we say about the kleene closure? Do we have examples for it too? I.e. can the closure of an undecidable language be decidable?

Also, can we generalize such undecidable classes?

We know that the halting language is undecidable. Let H be its binary encoding. We can also state that complement of H is undecidable. Therefore, union/intersection of H and HComp are $\Sigma^*$ and $\phi$, which are decidable.

StackExchange Computer Science Q#1549, answer score: 9