And when? Assuming you chuck two black holes together, on chocolate and one vanilla, remember they are just points in space. It is not inconcievable that if they came together on an intercept, but not head on collision where the distance between central points was less than the Schwartzchild radius of their combined masses they could co-rotate inside the larger black hole in some cosmic death spiral for some time before actually making contact. Perhaps millenia. We are talking about intersecting two theoretical points in space here.
So assume they meet at some point. (Not something I'm convinced can happen in any other than a theoretical sense) As they eat each other they release energy, most of which is retained inside the field but some of which leaks out.
The question I can't answer is "Is energy density in a volume equivalent to mass in the same volume?" If it is, then your matter/antimatter hole just converts to an energy black hole. If the answer is no, then as the energy goes up and the mass comes down the size of the hole dwindles and the energy escaping goes up to the point where as Hawking pointed out, small black holes explode releasing all their pent up mass as energy.