Arnuz: That's incorrect actually, increasing numbers of construction buildings have a penalty - diminishing returns kind of - that starts already from building number three. I'm sure of this for construction yards, I'd be surprised if the same isn't true for other buildings.
As a result, 2 planets with 2 constr. yards build faster than 1 with four, although 1 with four has a quicker "time to market" of the built item. So if nothing happens 2 with 2 are better, but if you're under attack it's the opposite.
No, you're wrong. There are no diminishing returns. What you're most likely seeing is that the game loses some points on the last object in a construction queue if construction points vs cost does not divide evenly.
If you have 4 regular shipyards, you produce 1 construction point/day, so regardless what anything costs, no points will ever be dropped.
If you have 8, you produce 2/day, meaning that if you build something that costs 5 and set each newbuild manually, then every 3rd day you will get 1 completed and lose 1 construction point.
But if instead you set a queue to build 2 or any multiple of 2, no points will be dropped and a build will be completed every 3 and 5 days.
This becomes very obvious if you edit construction to be twice as fast, meaning that advanced shipyards EACH produce 1 point/day, and then set up the primary planets for both empire and rebellion to start with enough energy that they can have at least 16 shipyards. Then that planet can finish cheap stuff like fighters several times every day. If you can pay the refined materials cost at least.
Which is why you also edit the map so that a few more sectors begins active and go ahead and use save followed by bombard military only on a few planets in the sectors you want, reload if you hit anything civillian.
Then to let the AI become a threat, when starting you have AI units set to half or less maintenance(beware of setting too low, or you might end up with an Empire with many dozens of ISDs and VSDs at start), followed by having a small ship(one with minimal bombardment value that is greater than 0) of yours in each sector you want to let the AI have, fly around to neutral planets and bombard civillian only, 1-3 pieces items destroyed on 1-3 planets is usually enough to make a whole sector go hostile.
Oh, and don't do this without having set all characters to not being able to go traitor, or you might end up with very few characters that do anything useful.