adjoin seems too similar to join ...

That's kind of why I liked it.

... which is unrelated.

join intersperses adjacent elements of a list with something.

This (in the simple 2 at a time variant) is often used to add adjacent elements of a list together -- ad-join.

A vote for overlaps (BTW, wouldn't "overlaps" be enough?).

Um. I thought about that, and still haven't reached a conclusion which is better.

In the singular, it sort of implies something only happens once.

But if you read 'overlaps' as a plural as in "there are many overlaps"; rather than the (*)???: "it overlaps", then it makes more sense I think.

Not sure what grammatical term applies here?

The nice thing about overlap(s) is it works for the variable width (depth?) of overlap also which adjoin doesn't.

I think I'm beginning to favour overlap(s) also.

But now I'm also thinking about overmap and mapover?

Well, mentioning the plural brings a new perspective...

For me designing and naming a good API is far more complicated than coding, since it involves human psychology.

Especially since sometimes good names are already used or even only reserved.

This brings a new concern, wasting an otherwise better used word.

overlaps could mean much more things and especially in plural it could indicate tuples bigger than just a pair of values.

The English grammar has no dualis alongside singular and plural like for instance Arabic, so only constructs like pair or couple indicate 2 tuples, and couples are normally disjoint. Maybe a construct with prefix "bi-" could solve this (think bigamie ;-)

Other than describing the tuple as a whole one could try describing the relation between two elements.

So my best guess ATM is "neighbor" but in singular(!). The neighbor can't mean anything else it describes a relation between 2 objects.

Words like next or link already have different meaning and bichain sounds artificial in my ears.¹

Otherwise I would consider successor or one of the synonyms like follower or one of it's synonyms like companion.

As soon as the definition is clear seeking through a thesaurus is of help.

updates

¹) bimap ?