Tweener

A contraction of the words in between.

1) It is an item or person that is in between two things.

Examples: a) Something that is small between two larger things. b) A person stuck in the middle between two sides of an argument.

2) The middle person of a threesome.

This task to do at work is a tweener. We don't have to do as much work as that large project, but it is not a simple job either.

Jane is always the tweener of that threesome.