Well each additional engineer you add will lower the build time by a smaller amount, but each additional engineer always takes the same amount of resources (assuming same Mk engie, I think).
In simple terms, this means your money will be spent most efficiently if you spread out your engineers. This will also maximize your total building capacity.
Example, one space dock.
Build time = 60m
One engineer, =30min
Two engineer = 24min
Three engineer =21min
Four engineer = 19min
Example, two space docks
Space Dock 1
Two engineer = 24min
Space Dock 2
Two engineer = 24min
The numbers do not represent the actual in-game build time, but they do represent the idea of diminishing returns. Remember, each engineer always costs the same amount of resources for each particular ship (whether its the 1st engineer or the 2nd engineer on a single cue). By doing a little math, one can see that building two units with a build time of 24min will produce units faster, all but the very short run, then one unit every 19min.
My message is not always coherent at the end, sorry.
Higher Mk engineers accelerate BP faster and repair faster, and since repair cost is based on per second, not on how much is being repaired, they are more efficient.
Does this clear up our the engineers work OP?