Having only one source of violent coercion is
  vital for a society to function properly

Is it a problem if the sources don't share jurisdiction, or don't disagree in areas of shared jurisdiction?

I mean, is it a problem if the French and German governments disagree, or if boxers hit each other, or if America decides to use drones to bomb some terrorist training camps in Pakistan, or if the tax authorities pursue offences the drug authorities are indifferent towards?