If the Earth split in half it would result in the immediate death and demise of all life on earth and a complete transformation of the planet earth.
If the Earth would to split in half it would be catastrophic and would cause massive earthquakes and expose the interior of the Earth to the vacuum of space which would lead to the loss of the earths atmosphere and magnetic field.
The resulting fragments when this happens would eventually cool though and become new and uninhabitable planets.
The initial split of the earth would cause massive earthquakes and immense seismic activity and shake the entire planet and destroy infrastructure and everything on the planet.
The oceans and the atmosphere would also be thrown into chaos if the earth were split in half and massive tidal waves would also occur and the atmosphere could also be potentially pulled away from the split of the earth.
The earths core and mantle would also be exposed to the vacuum of space and lead to a loss of heat and the planet's magnetic field.
The weakened gravity on the new, smaller planets would also cause the atmosphere to escape into space if the earth were to split.
Once the earth has split, the two split halves of the earth would also likely become 2 smaller and independent planets but be uninhabitable and all life on earth would be extinguished due to the immediate devastation and the lack of a stable atmosphere.
The new planets that would form from the 2 split halves of earth would be vastly different from planet earth and have unfamiliar atmospheres, temperatures and gravitational forces.