Comorbidity

As with many of the non-disease, non-treatment MeSH, Comorbidity is used inconsistently by indexers. Here are some keywords to help you:

"Comorbidity"[Mesh] OR comorbid*[tiab] OR co-morbid*[tiab] OR co-occur*[tiab] OR simultaneous*[tiab] OR superimpos*[tiab] OR coexist*[tiab] OR co-exist*[tiab]

If there is a sequence, that is if the patient first had one disease and then acquired another, you can also use:
preexist*[tiab] OR pre-exist*[tiab] OR antecedent[tiab]

Of course, your best bet is to combine the first disorder with the second disorder, without using any of the above. This is for cases where you're looking broadly (e.g. all of the diseases which can be comorbid with a particular disorder), or when there is another combination of the two terms that is cluttering up the search, for instance if you're searching for people who are addicted to two substances, and come across a lot of articles about treatment of patients who each have one of a variety of substance use disorders.

For the specific instance of a mental disorder and a substance abuse disorder, there is:
"Diagnosis, Dual (Psychiatry)"[Mesh] OR "dual diagnosis"[tiab] OR "dual diagnoses"[tiab]