With two well-separated black holes it's reasonable to talk about each of their rest masses, and the potential, just like the apple. But as they get close and merge these ideas are hard to pin down, and stop being useful. Their shapes get blended together, and their horizons unite into one sphere, and for a while this wobbles around a lot before settling down. Some of the energy of its wobbling around departs as gravitational waves. Once it's settled down you can meaningfully talk about its rest mass again.