[Disclaimer: I work at Google, all words my own and not representative of anyone, etc.]
There's no such thing. Let's stick to software since it's what HN knows best, but it's a universal truth.
Merging two companies: you take the two sides and they keep running. You probably need some extra accounting work to make sure the mandatory reports from each side get combined, but that's all you have to do.
Splitting two companies: oof. If you split ChildCo out of ParentCo:
* You need to hire all the roles that weren't part of that organization before (HR, legal, compliance, etc.)
* You need to register this new entity in all the countries it operates and/or has employees in
* Technologically it's somewhere between messy and impossible. Now you no longer have Google's build stack or monorepo. You have to rewrite everything. You can't just move some VMs to a cloud provider of choice