A common setup: 2 quantum gateways, with hives in between. The gateways make RAS SACUs. The hives assist both of the gateways, alternating between them. This way, the hives are almost always active.
If there was only 1 gateway, during the "roll-off time," the hives would be idle, so you would not able to use all that build power as efficiently.
But what if you are not Cybran? Can you achieve the same thing, with normal engineers? You can, actually.
You can get engineers to behave this way (alternating between supporting 2 quantum gateways) simply by placing an "assist" order on the ground near the gateways.
You can see this in replay #12819570
I selected all of the engineers and gave them a single (shared) assist order that was placed on the ground near the 2 gateways. At first, about half supported the left and half supported the right. Soon after, they synchronized and began alternating between left and right.
In replay #12819438 I did something a little more complicated. I picked a "lead" engineer who had an assist order on the ground. The other engineers all assisted this lead engineer. That guaranteed that all of them only ever supported one gateway at a time (whichever one the lead engineer decided to assist).
I suppose you could also use this technique, if you planted some engineers between 2-4 factories, and gave an assist order on the ground, they would alternate between supporting different factories rather than being idle during roll-off time (if they were ordered to assist just 1 factory). Which could help to more efficiently produce units, especially at the t2/t3 stage.