So, what's important is not that there are em dashes, but that what is between them is regarded as an afterthought or a parenthetical, a kind of whispered aside in the course of formulating the sentence subject.
The problem is that dashes can also be used to emphasize what they set off. Unless there is some phrase like "to some extent," as in the Chicago example, to signal that it is a parenthetical, it is unclear that that is how it is intended.
In the Egyptian example that Ahmed Imam Attia has asked about, there is nothing that indicates that "and that troublemaking friend of his" is intended as an afterthought rather than an emphasized conjunct, which it appears to be.
If it is an emphasized conjunct, the addition of "both" will be possible within the predicate, and that addition will render the singular verb not just unnatural or unidiomatic, but totally ungrammatical.
(1a) Walid—and that troublemaking friend of his—have both arrived.
(1b) *Walid—and that troublemaking friend of his—has both arrived.