"I think 'twin' as used here is clearly performing an adjectival function on 'cities.' Would 'both twin sisters' suggest 4? I think not. Although in Newfoundland, some people use 'twin' (singular) to refer to both siblings, as in 'Flopsie and Mopsie are a twin.'"
Clearly, alternative interpretations exist. We do, in fact, mean only the two cities that are commonly referred to as "the Twin Cities", although we would gladly perform for any other cities (or even small towns or unincorporated rural areas) that might be interested in having us do so.
We thank the heads-up visitor from Chicago and equally shrewd Newfoundlandian, and we truly apologize for any confusion that all this may have caused.
Editorial update:
Subsequent to our publishing the preceding discussion, a concerned Canadonion brought to our attention the fact that a native of Newfoundland would most properly be referred to as a "Newfoundlander" rather than a "Newfoundlandian". It was also suggested that if we wanted to be mean, we could simply call the person a "Newfie", which, despite its suggestion of cuddliness, we would steer clear of for fear of alienating inhabitants of that most recent addition to the Union Of The Maple Leaf. All of this information was provided to us by a Haligonian, i.e. native of Halifax, which is in itself an interesting term and which probably has nothing at all to do with "halitosis", although this editor must confess to having no experience with genuine Haligonians.