What happens if a black hole is produced inside another black hole?
From the no-hair theorem, black holes can have only three properties: mass, angular momentum, and electric charge. From this we'll see the original, supermassive black hole gain the mass, angular momentum and electric charge of the ingoing star. The black holes will merge (already merged actually, since one is inside the other). The singularities will merge too (all world lines, i.e. paths a particle can take, lead to the big singularity in the supermassive black hole). 

From the outside, we will see a more massive black hole with different angular momentum &amp; electric charge, and that's it. We can't see the star go supernova because anything that happens inside the event horizon is invisible to us.