I too have wondered about 2 big diesels on a small train which one smaller geep could handle.

There clearly are several important variables. I can think of a few:

The smaller geep can't be used on almost any train 24/7 and thus will sit hours without work.

The smaller geep would have to run long nose forward about half the time or be turned often. Turning is costly in terms of labor and time and there are far fewer turning facilities these days. Are there agreements about running long nose forward? Safety issues?

The small unit being an odd ball would have different maintenance procedures and parts inventory.

Are the fuel costs that much higher with the two big units instead of the small one doing the same local freight work?

Not having any numbers to do the calculations necessary to answer the above issues when the RR economists have all the numbers, I am sure they are minimizing cost and maximizing revenues.