Cfrtjryk

I know that diaeresis is used to show that two adjacent vowels are not a diphthong but should be pronounced separately, as in naïve or Zoë. Is there an equivalent mark or format in current or historical use that shows that a pair of consonants that usually form a digraph (e.g. "sh" or "th") should be read separately?

Cases where a word is is made up of identifiable parts are easy to deal with. One can do nothing and rely on the reader's understanding of the separate morphemes (e.g. knighthood) or with true compounds one can put in a hyphen, e.g. pot-hook.

However there is more of a problem when transcribing a word or personal name that comes from an unfamiliar foreign language (so the reader is unlikely to know its spelling conventions), is not a compound, and yet contains a syllable ending with "s" or "t" immediately followed by a syllable beginning with "h", or or another easily misread combination.

Right now I can't think of any words either from English or a from a foreign language which present this problem, but among all the vast multitude of proper names and languages in the world that sometimes need to be written in English it must sometimes occur. It also would come up in transcribing fictional constructed languages so as to sound "alien" yet still be easily readable. In fact my question here was inspired by this question on Writers' Stack Exchange , in which it was asked how to represent words from a fictional language that would be likely to be mispronounced in English.

Inserting a hyphen into a word that is a single unit of meaning seems wrong. Inserting an apostrophe might be better, but an apostrophe suggests either a glottal stop or the marking of omitted letters, neither of which might be present. I seem to recall once seeing a full stop placed between letters to show this but that might have been a quirk of an individual writer. Use of the International Phonetic Alphabet, explanations in brackets, or asterisks all disrupt the flow of reading.

Is there an existing convention or a better solution?

Added later: Some real life examples of words whose pronunciation would be clearer with a consonantal diaeresis:
- posthumous
- shorthand
- Mathias (German proper name)
- Kuthumi (name of a nineteenth century Indian mystic)
- methemoglobin / methaemoglobin / methæmoglobin (medical term, in which the prefix "met" means "change in")
- Ishak (Arabic proper name).
In practice with the exception of the occasional hyphen these words seem to have no orthographic device to mark the correct pronunciation, thus answering my question in the negative, unless there are counter-examples I haven't yet met.

Maybe you are asking for a diacritic.

How about this for example?

knightḧood

Other symbols that may be useful:

ḥ

ḫ

You can find their specifications at http://graphemica.com

If you are writing a scholarly article, all you have to do is define what it means before the main text.

Definition of diacritic in English: noun

A sign, such as an accent or cedilla, which when written above or
below a letter indicates a difference in pronunciation from the same
letter when unmarked or differently marked.

Oxford Dictionaries

Distinguishing between unionized (being in a union) and unïonized (not being ionized) is an example where a consonant/vowel diaeresis mark seems to be genuinely useful. I can't think of a non-contrived consonant/consonant equivalent though.