Apologies for re-opening this old thread, but as an industry insider, I do know how this works. I hope this helps someone, as I came across it whilst looking for something similar.
1) Yes, it is possible for 1 "import" meter to have 2 MPANs. This occurs when the site has (or had) separate heating and power/lighting circuits from the meter. In most properties the power goes from the meter to a fusebox, and at that point the separate circuits for power, lighting, cooker, showers etc are connected. However, if a heating circuit comes directly off the meter, and consumption is recorded on its own register, then that'll probably have a "related MPAN". It was set up this way because there is consumption on two separate registers (or combinations of registers) concurrently - i.e the heating can be on whilst the lights are on, AND the electricity settlements system was not set up to deal with this "properly". Having 2 MPANs is a workaround (that saved a bucket load of industry costs for a small proportion of meter points - dont think that "saving cost" is a bad thing - we all pay for those costs in our bills, so it was a good thing). This all started in the 1970's with Heatwise, Warmwise, Economy 10 tariffs to encourage electrical heating when storage heaters were the new thing. The E7 part of the meter takes the usual lighting and power (so you can run your tumble dryer at 3am on lower rate energy), and the "unrestricted" part handles the heating - this is usually controlled by a timer so its actually only consuming overnight, and sometimes has a "boost" option for use at other times of day.
2) Because the MPANs are "related", they should be offered with a tariff that supports the configuration. Many smaller suppliers dont offer these tariffs (some of them quite frankly dont have a clue what they are - they just offer unrestricted and E7 tariffs). Those small suppliers were given a dispensation from Ofgem, as the cost of supporting such complexity for so few sites was not considered economic (Ofgem is made up of economists).
3) It is possible for two separate properties can be charged off the same meter, but NOT under 2 different MPANs. The two MPANs will be related to ONE supply (albeit two circuits, in one property). If a building is converted to flats, its possible there may be one meter, and an arrangement to share costs (usually when the landlord remains responsible for the whole consumption, and sub-properties pay the landlord. There is also the possibility that the "main" meter remains in place and a "sub meter" is installed: the first property gets billed on the main (total) consumption minus the sub meter, and the second property gets billed off just the sub meter, but those are so rare.
4) See the answer to 2 - its possible the Supplier doesnt understand what you've got or cant support it: they SHOULD know, and if they cant support it they SHOULD not take on your supply (you can complain to Ofgem if they do). No - you should not be charged for both tariffs separately, but if its just consumption, then you're paying for what you use. IF they are charging you 2 sets of standing charge, then see above - complain to Ofgem and the Supplier will have to compensate you.
MissDaisies - sorry, but most of your answers are completely incorrect.
Surfer01 - I think you are mixing up "unmetered supplies" (landlord lighting - e.g. hallways in blocks of flats, remote water pumps in the middle of fields running 24 hours a day etc) with "unrestricted". Unmetered means exactly what it says - there is NO meter, and there doesnt need to be one because the energy company knows how much is being consumed (fifteen 60W bulbs running 14 hours per day uses a predictable amount of energy), or its impractical to install and read one (a locked pumping station in the middle of a field is not only remote, but you dont want any Tom Dick or Harry wandering in there and contaminating the water supply). This will NEVER be applicable to a standard domestic property. Unrestricted mean there is no time constraint on when the meter register records consumption (i.e. it records consumption 24 hours a day, even if theres another timeswitch than constrins the consumption to a known period.
If your property has two MPANs, BUT you no longer have a separate heating circuit, then you can get your Distribution business to de-register one of the MPANs (your energy Supplier should be able to ask them on your behalf, but as many dont have a clue (see above), you can go direct to the Distributor and threaten a complaint if they dont and its preventing you from changing to the Supplier of your choice - as a Regulated business, they'll not want an Ofgem complaint on their books.