Fission doesn’t happen because we cut atoms in half. Fission happens because we blast enriched uranium with neutrons, the uranium absorbs a neutron, gets too heavy, and falls apart.
I mean think about it. Atoms are surrounded by a negatively charged electron cloud. Pushing 2 atoms together would be (sorta) like trying to push the like poles of two magnets together.
Sure, but you can also rip off electrons from atoms by rubbing them or bending a piece of wire. The energy needed to trigger fission in uranium is less than a picojoule, it just needs to be focused enough to knock away the part of the atom, which is why neutrons are the most common way.
Here is a chart with the rate of fusion for two hydrogen atoms at various temperatures.
https://en.m.wikipedia.org/wiki/Nuclear_fusion#/media/File%3AFusion_rxnrate.svg
This chart bottoms out at a few million degrees, since the probability is extremely low.
