In practice, I would point you to another term of poly-centric law and poly-centric governance. In some respect I'd say that we work within a somewhat poly-centric legal system in the abstract but when you zoom in to a local level it is still based off the monopoly of a territorial claim. It is not too far fetched to see how poly-centric law works today, but to move to the truest form is not in practice, but in effect would offer a market for governance. I would label myself as a "Libertarian" to most people I speak with but if were really talking I'd identify as an Anarchist. But Why should I and a majority of like-minded people get to tell a minority their government cant pay into a 'universal healthcare' if they want? and likewise (and more reality based) should I have the majority tell me I have to pay into a 'universal healthcare' government. What if we didn't vote for people (with power for a timespan) and instead payed them directly. Well the idea is that you'd pay your government truly voluntarily and receive it's benefits, and I would to mine. And where these two governments agree on things they form collations like lets say to fund a Parks department (So I can still have nice hiking trails) and where our two disagree with other governments like lets say i.e. (National Conservative Government) who pays for a Department of Defense (which is offensive) and outlawing it's own citizenry abortions We'd have no coalition, and both our citizenry of your and my government pay no for their war (and if that were the case like with the overarching unpopular "Iraq War' the funds collected by the "Governments /Parties/" would not be able too, or at the very least I would have to pay for the military who invades and nation builds foreign soil.
I'm rambling, but how would it work in practice, just like now except more market effects (and Markets for Governance is the most critical one which is monopolized) And most libertarian types will point to the closest historic examples as mentioned like medieval Iceland and Catalonia (Spanish Civil War times, and was a Left-Anarchism)