This rule is highly speculative but based on patterns observed in Tolkien’s original Adûnaic words.
[e] and [o] are always long, unless you have pair of either with no different vowel in between, then you speak each one in short form since together they make a long vowel.[1]
one or more consonants in between them does NOT make a difference, you can have a consonant cluster like in igmil and the rule will still apply to [e] and [o].
If there are two [o]s together, each is then reduced to short form, since together they make a long [ō] that just happens to have a consonant inside it (for example, obroth, “fore-wake”).
If there are two [e]s together, treat each one as a short vowel, since together they make a long [ē] (for example, izreneth, “beloved woman”).
Note this does not apply if there are two long vowels of different kinds next to each other, such as an [ē] and an [ō], or an [ō] with a [ā], or any other combination.
[1]. This does not apply if one of the two long vowels in question is the ending vowel for a Weak-II/Strong-II word, such as lôkho, since it does not suffer from vowel congestion.