History shows us that diabolical rulers seem to favour huge big expensive heavy impressive useless overdone impractical weapons over the more humble and nimble ones.
Take Germany in WW-2. It took 300,000 manhours to build a Tiger-E, where as the Russian threw together a T-34 in under 30,000. Sherman too was completed in roughly 30,000 manhours.
End of the story: 1300 Tigers versus 30,000 Shermans. I know who wins, no matter how "invincible" you make that Tiger.
Even though the Tiger-E was impractical, clumsy, technically unreliable, they designed the even bigger, heavier Tiger-B, and went overboard with the Maus, a behemoth of 250+ tons in weight, with absolutely no value on the battlefield other than a barely mobile bunker.
The Germans even drew up plans for tanks in the order of 1000 and 1500 tons, mounting battleship turrets on them boasting dual 28cm guns.
But, with the DS as a weapon, it's not that big bad gun inside that's the point, it is how much fear it can instill.
The DS wasn't something of a huge offensive weapon, it was meant to be a deterrant, don't mess with the Empire or we'll come and get medieval on your asses.
In the game Rebellion/Supremacy using a DS is perfect to submit systems to bow to your rule, playing the Empire.
But they consume huge quantities of men, time, raw materials and what else.
However, the Empire seems to have that, as the are the GALACTIC Empire, and that means, we've got it all, and we just have no clue what to do with it.
So, let's make this super-weapon, the Uber-gun.
Let's take that Maus into the picture, a massive, huge, thick armoured and heavily armed monster of a tank. It's pretty much stuck to where it stops moving, so... why don't we just drive around it, really fast?
This applies to the DS as well, even though it can travel through Hyperspace, in ANH it took quite a while to clear Yavin in order to shoot at the Rebel base, making the DS a sitting duck for most of the time.
In ROTJ, the DS seems capable to some quicker firing-rate, but still, it is effectively tied to where it is, Endor, and can't function as the terror they've constructed.
Granted, that was the Emperor's plot, but still.
The first DS being a solitary DS on paper sounds pretty impressive, but as long as it's not around, it's not a threat. As ANH demonstrated, the Rebels could move around quickly, leaving deserted bases behind like the one on Dantooine, and also, the swift evacuation of Hoth.
I don't think the Empire could toss the DS around in space as fast as the Rebels could relocate, or even, disperse.
So for a DS to be effective, it is imperative that you have more than 1. That way, you can be everywhere where there is trouble.
Think about this, you're aboard a B-17 on your way to Berlin, and this beautifull Mustang with ít's 6 browning's is escorting you. A Messerschmit appears, the Mustang speeds away in pursuit of the Messerschmit. Meanwhile, an additional 2 Messerschmits shoot down the B-17.
So, in essence, there's a lot to say in favour of the idea that a second DS was started together or some time after the first one was started.