A myth that needs to die.
fun - gi - ble -- /'fənjəbəl/
"... able to replace or be replaced by another identical item; mutually interchangeable." Google
One engineer, is not interchangeable with another!
Project Resource Planning
The Holy Grail
The loss of any one software engineer is easily remedied with addition of another.
"Pat has decided to seek new opportunities and will not be working on the project from now on. Sam will be taking on Pat's responsibilities."
It never goes this smoothly.
The problem is not that one of these two people are better, but that there are differences between them.
Choose Your (Our) Own Adventure
We have a few paths that we can travel to explore the reasons why 'Fungible Engineers' is a myth, which should we take?
Do you want an electrical engineer designing the structural components of a large building? Or would a structural engineer be better suited to the job?
Difference Is Good!
Heterogeneous vs. Homogeneous
Both are good but they are orthogonal to each other; you cannot have both.
You say that you want engineers that are interchangeable. I say that you do not. Design by committee is a recipe to not change the status quo and continue to produce what has been produced in the past. Creativity is the work of few and unique people. Foster that creativity - that innovative spirit - by encouraging them to continue. Do not suppress them with the idea that they can not be successful on their own.