All you need to do is to ensure that there's not only the two halves involved. As long as some substantial mass escapes completely, it basically doesn't matter

All you need to do is to ensure that there's not only the two halves involved. As long as some substantial mass escapes completely, it basically doesn't matter if the rest is split or not - conservation of momentum guarantees a substantial change in trajectory (at least relative to the accuracy requirements for Earth impact).