If you simply mean a continuous urban area, then New York City arguably extends well beyond its geographical boundaries into both other parts of New York and into New Jersey. If we mean administratively, then no such thing exists within the US.
True. But not entirely relevant. The fact that none exist iOTL doesn't prevent their existing in some other TL. It suggests low probability, it doesn't prove impossibility,
There's a long series of precedent in US law, however, that limit incorporated municipalities to one state - the federal-state-local system pretty much requires it. Crossing state boundaries for "local" needs is possible - the Port Authority of New York and New Jersey being an obvious example - but they are generally pretty rare and negotiated through what amount to joint power authorities between states, not between municipalities in two different states. It is not the Port Authority of New York City and Newark, for example.
1) So we'd have to change precedent, and/or find a good reason to over turn it. This DOES make the task much harder. I still believe it's not impossible. It may be that doing it long before 1898 would be easier.
2)the very fact of things like the Port Authority existing suggests (but again, doesn't prove) that the states COULD, if they wanted work out a 2 state city.
Probably not once the US becomes independent, unless you have a way for the federal system to include interstate municipalities in some way. It might not be too hard to establish a precedent for it in the early does of the US though, since there would be and still are lots of cities that span multiple counties. Maybe it's set as a compromise for the Toledo War ending in Toledo being divided between Michigan and Ohio along the Maumee but still having a single city government?
Certainly, after the US becomes independent. Even after the ratification of OTL's Constitution.
Interesting idea for the precedent to be set in the territories as they become states. A multi-territorial city might be easier to set up than a multi-state one, and negotiations leading to statehood could keep that city united. Then, when NYC amalgamates, the precedent COULD lead to part of NJ joining,
A multistate unitary municipal authority isn't possible under the current Federal system. You'd have to change the constitution.
Wrong. The constitution is totally silent on the matter of cities. (Well, aside from the special case of DC.)
To the extent that forming a multiple state city would require the approval of both (all) states involved, yes. But that approval is surely possible. (Again, even if it is less likely than not.)
Surely if the hidebound UK can recognise urban realities and create the GLC uniting the urban sprawl of London subsuming Middlesex and bits of Surrey etc then the far less hidebound Americans are capable of doing something similar with New York? Admittedly it won't work if there are long standing "animosities" to consider (Humberside and Tyne and Wear for example).
Quite.
It's near impossible. To do so would either require a complete reworking of the Federal system, in which municipalities and counties have no existence independent from the state governments, or for both New York and New Jersey to willingly cede their most economically productive areas to a new state of New York City. Which would never happen in a million years. Frankly, there's no reason for it to happen, either. No one really gains anything by merging the core of the NY metropolitan area into a single municipality.
Nope. In fact both NY and NJ might allow the expansion to a multi-state NYC precisely to INCREASE their most productive areas.
You would definitely have to get both states to agree, and you might need some formal agreement 'treaty' between them. But it is most certainly not impossible. Improbable, I'll grant. Impossible, I won't.
That's exactly my point. Because of the fact that municipal and county governments are integral to and dependent on state governments, they cannot be interstate in principle.
But once you set up a multi-state city, it's dependent on both, not just one. It would be interesting to see how such an arrangement would work/be worded, but if the agreement between the states were 'perpetual', it might be difficult, at best, for a single state to abrogate the agreement.