Isn't this why microservices are a thing? :).

More seriously though, it's part of skilled project management to organize work in such a way that seniors don't need to constantly collaborate to stay in sync.

What one programmer can do in one month, two programmers can do in two months.

It's skilled software architecture. Some layerings of the problem into decomposable pieces allow for less communications and some layerings of the problem require more communication. The emergent but desirable property of more parallelizable is dependent on details that are small and subtle. Project management cannot possible guarantee this. The outcome of the architecture, how much do people need to communicate, is an input to the project management.

Your “skilled project management” is my “unnecessarily complex process.” There’s a reason collaboration of this sort is used in real engineering disciplines: it’s because it’s more efficient than having some middleman (and/or process) mediating all the communication.

I didn't mean middleman/process mediating communication. By "skilled project management", I meant structuring tasks and setting priorities in such a way as to minimize the required amount of communication - particularly, synchronous communication between the workers. An hour (on average) or two per day replying to messages and doing code review, an occasional meeting every other week - sure. But if your senior devs are spending most of their time each day in meetings, then either you're running a bootcamp, or something is very wrong.

We are not microservices. People in a team have a lot in common, they do systematically the same type of work albeit in a different context and they can be a lot more useful to each other when they communicate seamlessly instead of at scheduled time slots.