Well, several things come to my amateur mind; first, where does it say that something falling into a BH gets to lightspeed? For a supermassive black hole, IIRC, something can drift in relatively slowly. It's only the escape velocity of a BH that is lightspeed.
Originally Posted by Sparki
Second, it's only the relative or apparent mass of an object that changes as it approaches lightspeed, not the rest mass.
Third, BHs do not have infinite mass, as we've been able to measure their mass in terms of multiples of Solar mass.
Fourth, we have no idea what actually happens inside an event horizon, as it's impossible to observe the inside of one, but multiple black holes too close together are thought to merge-- violently. As most BHs we've observed absorb matter yet do not constantly display signs of mergence, there probably are not other black holes hiding inside the event horizon.
STARGAZING: All I see are the lights of a billion places I'll never go. --Howard Tayler, Schlock Mercenary