Not quite sure what you mean by 'at its heart'... - do you mean that the name of the town contains the names of two other cities or do you mean that within the boundaries of the 'town' there are 2 cities? - If the latter I would guess it is Greater London with the cities being the City Of London and (Westminster?)
If the former I would say probably Londonderry - the name including London and Derry - but I thought that place is a city with 2 alternative names.