Does the “in” imply multiplication, in which case split in half is correct, or is it division? It sounds like the latter to me, but I’ve heard it used both ways.
They are both correct. If the focus is on the fact that I now have two objects, whereas before I had only one, I’d say split in two; if I want to stress that now each object is half the size of the single object I had before, I’d say split in half. In each case, though, the emphasis is so subtle that it would probably escape the conscious perception of the hearer, as indeed, the stress probably was not consciously calculated by the speaker.