To connect to multiple Kubernetes clusters with kubectl, I like to define aliases like this:

alias rancher="kubectl --kubeconfig ~/.kube/rancher.config"
alias work="kubectl --kubeconfig ~/.kube/work.config"
alias customer="kubectl --kubeconfig ~/.kube/customer.config"

Those aliases allow me to write things like rancher get pods --namespace some-namespace without worrying the wrong context is active. Using multiple configurations - one for each cluster - seems to be easier to manage since most clusters allow to download a ready-to-use configuration file. Instead of mangling them together manually, I just specify another alias whenever I get to work with another cluster.