two black holes coliding would not cause a big bag in any way shap of form. First off im assuming you are talking about a situation in witch the event horizin of one of the black holes extends to emgulf the singlarty of the other black hole if it did not grow to this point thay would simply orbit around each other much inthe same way binary star systems do. once the event horizion engulfed the singlarty you would not have two black holes but one the black hole with more mass would simply add the smaller black holes mass to its own . you would just end up with a " bigger black hole" ie its event horizion would expand reltive to the abount of mass the in the smaller black hole it just devowered.