In a merger you can take as long as you want to go from

1. Two separate companies except at the end of the quarter we add their revenue and expenses together in a spreadsheet to transition to

2. One fully integrated organization

And usually you are becoming more efficient and saving time and money as you integrate.

Splitting a company needs to happen quickly or you'd get all sorts of weird effects where coworkers are ostensibly competitors whilst sharing resources during the transition. And you have to expend a huge amount of effort. Just a couple random complex systems that need to be untangled off the top of my head: physical property and leases, IP space for every IT service you run, multi-year contracts with every vendor from janitorial to SaaS, multi year contracts with customers depending on how the split goes, and of course all the intermingled finances and HR and spreadsheets every company in the world lives on. I'm sure there's thousands more considerations.

I agree antitrust is a big problem that needs to be solved. But "it should be the same amount of effort to merge and split a company" is just fantasy.

What makes splitting a company out difficult isn't (directly) a financial or paperwork burden - it's that tightly integrated systems are very difficult to untangle. There is nothing analagous that could be introduced in the merger process. You could add a mandatory delay, but that's not making it "as hard", it's just making it slower.