It depends on the situation. There are cases where a senior engineer outperforms three or more warm bodies and cases where two warm bodies would be much more preferable, it depends on the content of that job.

And I have witnessed a case where simply removing "one senior engineer who has worked on this particular software for years and therefore knows his way around it" was a key part of finally getting that software on track, as he was providing literally negative value.

Also, relying on particular single people who "know their way around" some code is an organizational smell - if you have such a dependency then that's a management problem/business risk that needs to be fixed. You do need shared ownership and people who can replace each other, and (except for the very short term) a better engineer will be more productive than a worse engineer who has been stuck on maintaining that code for years.