No cannot use the same key name for key/value pairs in YAML dictionaries. Use different keys. To schedule a pod to nodes labled with node:super or nodetype:duper:

spec:
  nodeSelector: { node: "super", nodetype: "duper" }

In YAML

martin:
  name: Martin D'vloper
  job: Developer
  skill: Elite

is the same with:

martin: {name: Martin D'vloper, job: Developer, skill: Elite}

If it can be one line, it is easier to manipulate.

The better way is in using something like this:

  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
        - matchExpressions:
          - key: kubernetes.io/hostname
            operator: In
            values:
            - node1
            - node2

The nodeSelector on a PodSpec is just a map[string]string (instead of the more featureful NodeSelector type that is used in the NodeAffinity object).That means that the key "app" can only have one value, and it ends up being overwritten as "node2".

You can accomplish the scheduling between node1 and node2 by applying a common label (e.g. scheduling-group: foo), and then have your replicationController use that label as the nodeSelector.

Yes, you can.

If you want the pods to be scheduled to either node1 or node2, you may label both nodes with the same label(s) (for example, app=node), and then add app=node as your nodeSelector. You can add as many labels as you want, but like CJ mentioned, nodeSelector is a map with key and value pairs so it can't have the same key with different values.

The pods can be scheduled on any of the nodes that satisfy that nodeSelector.

For more information, read Assigning Pods to Nodes. Note that there's a new feature called nodeAffinity that you might also want to try.