The so-called best single adjective you could use for this is equidistant:
1 : equally distant
// a location equidistant from two major cities
In the example sentence, that would be:
The two equidistant cities are included in our comparison.
However, I said "so-called" because there are two problems with equidistant:
- Normally, two or more things are equidistant from something else.
- That they are equidistant doesn't necessarily imply that there is a great distance between them. You could, for instance, say that the I and M keys on a keyboard are equidistant from the K key, even though they are not far apart from each other at all.
So, while the meaning of equidistant might be conveyed in the right context, the adjective entirely on its own doesn't convey that meaning. But it's not clear what the context of the sentence in the question actually is. If the comparison being discussed talks about things with a great amount of space between them, then the two equidistant cities could work.
If an adjectival phrase is acceptable, you could say something like this:
The two mutually and greatly distant cities are included in our comparison.
The adjectives mutually ("directed by each toward the other or the others," as per Merriam-Webster) and greatly combine to express exactly what you want to express, and the sentence doesn't need to be rephrased.