A better example would probably be "ij" in Dutch. That's definitely considered a single letter, as words starting with ij in Dutch are capitalised IJ. Though there are glyphs for IJ /ij already in unicode.
We also have "ei", which sounds the same and was invented to annoy people learning Dutch. Then there's "oe", "eu", "ui". And just to fuck even more with people learning the language, we have "au" and "ou" which also sound the same. Oh, and "ch" and "g".
Hans Brinker, the inventor of the Dutch language, famously would toss a florijn to decide between using ei/ij and au/ou, as he was not fond of foreigners. He's mostly known for saving our country though when he plugged a hole in a dyke with his finger (yes, I know what you're thinking, and no, we do not appreciate your dirty minds making light of this heroic act).
Just did some googling. Turns out that unlike the other combinations, capitalizing both letters is mandatory for 'IJ'. TIL...
"IJ (lowercase ij; Dutch pronunciation: [ɛi]) is a digraph of the letters i and j. Occurring in the Dutch language, it is sometimes considered a ligature, or a letter in itself. In most fonts that have a separate character for ij, the two composing parts are not connected but are separate glyphs, which are sometimes slightly kerned."
(and equivalent in the Dutch Wikipedia article)
It could, mind, I don't know dutch. But in french "œ" (which has a ligatured character as you can see) is canonically equivalent to "oe". It is not a separate letter of the alphabet even though:
* many words should not be written with the ligatured form
* many words should be written with the ligatured form
* it has a different pronunciation than the base form