OpenShift will only give you access to one core per application instance (per "gear").
If you want to cluster Nodejs on OpenShift, I'd recommend using OpenShift's HAProxy tooling to scale up and down.
I wrote up a few notes on this topic here: https://www.openshift.com/blogs/10-reasons-openshift-is-the-best-place-to-host-your-nodejs-app#scale