Combining double macron

It spans a two-letter unit, not just one letter.

In the text sequence, ͞ goes between the base letters: o͞o or a͞e. Its combining position follows the first letter, while its shape reaches across the second, joining the pair visually as one pronunciation unit.

A proposal submitted in 2002 noted use of this mark in English dictionaries and teaching materials. It described the mark as a way to encode a bar spanning a two-letter pronunciation unit. The proposal grouped it with a double breve and a double macron below the letters, alongside earlier encoded marks for a double tilde, inverted breve, and rightward arrow below.

How do I type combining double macron between letters?

Insert it between the two base letters, not after both.

For o͞o, the mark is the middle character in the text sequence. It follows the first base and visually bridges the next. Correct text can still look wrong: bridging marks need special font positioning, and unequal letter widths can leave the bar off-center. A font with dedicated bridging support is the practical fix.