Node.Js - multiple workers on 1 cpu? (Cluster)

The situation is as following:

I have a node.js server with a script which takes pretty long before it finishes. The script is getting an ID, looks up in a database which pictures belongs to this ID, and then it cache's the images and once all images are cached, it finishes.

Now the problem is that its possible there are 2 or more people at the same time using this feature. And once there are multiple people trying to get all these images, the images are combined to eachother and person A gets the pictures of person A + B. and also person B gets the pictures of A+B.

Now i know that a worker require's 1 cpu. i edited this so i can have multiple workers on 1 CPU. But they only switch from workers when the CPU usage is really high. I want to switch workers when someone is already busy with getting these images, and someone else is trying to also get his/her images. (which are different for every person.)

How can this be done? Because the cluster only switches workers when the CPU usage is high. Or did i understand this incorrectly?

via Alex