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3 flats in the building, 2 of which are leasehold with a share of the freehold to the building. The other flat is leasehold without a share of the freehold.
So 2 different maintenance relationships exist. One with the other freeholder, and one between the 2 freeholders and the leaseholder.
neu-
Unless there's something very weird going on, all three flats will be the same leasehold tenure. The two people who own two of the flats also coincidentally own half of the freehold each. Presumably they'd sell you that share at the same time they sell you their flat, but they don't have to.
You'd agree what maintenance needs doing with the other freeholder and then send a third of the bill to the other leaseholder. This isn't terribly different to if all three of you were freeholders.
grams
You mean each flat has a separate leasehold and the freehold for the building is shared or each flat has its own separate freehold for half the building? The latter would be unusual.