Short answer: Bipolar disorder is probably not composed of two comorbid illnesses, but it may be on a continuum that includes some depressive disorders.
This is a good question, though it does convey some confusion associated with this diagnosis that should be cleared up.
The first confusion I think is the idea that "depression", "mania", and "bipolar" are disorders. Depression and mania are not disorders - they are symptoms. And bipolar is a "spectrum" disorder - a collection or continuum of similar diagnoses, that all feature at least one manic or hypomanic episode, and in most cases at least one depressive episode.
It is true that depression without mania is one of the key symptoms of a variety of depressive mood disorders (such as major depressive disorder, sometimes referred to as "unipolar" because of the lack of manic episodes), so it may be confused with a disorder. However, this is not the case for mania, so diagnostically bipolar disorder would not be confused with a comorbidity of 2 disorders - depression and mania - because mania is not a disorder.
Mania rarely presents without other symptoms:
Although bipolar disorder is by far the most common cause of mania, it
is a key component of other psychiatric conditions (e.g.,
schizoaffective disorder, bipolar type; cyclothymia) and may occur
secondary to neurologic or general medical conditions, or as a result
of substance abuse.
Presumably, a standalone manic episode may be sufficient for a diagnosis of bipolar I disorder or a "not otherwise specified" (NOS) diagnosis, depending on the severity and other symptoms involved. So diagnostically, "mania" as a standalone disorder is actually a sub-type of bipolar.
The second confusion I think is the idea that a bipolar diagnosis in some way precludes the symptoms of depression and mania from having separate causes.
Historically, bipolar disorder was not seen as 2 separate illnesses because (1) it was believed that mania and depression were mutually exclusive in time - as 2 extremes (poles) of a single mood continuum, and (2) mania rarely presents without depression. The assumption of mutual exclusivity of mania and depression was called into question by the occurrence of mixed-state episodes - a condition that features symptoms of both mania and depression simultaneously.
Ultimately, many psychiatric diagnostic criteria are encumbered by some historical precedence, common clinical presentations, how patients perceive their symptoms, and also some political influence, as diagnoses are associated with healthcare subsidization, stigma and prejudice, and explanatory power for the patient. So diagnostic criteria should not generally be confused with an understanding of common cause.
The cause of bipolar spectrum disorders is not well understood, and the question of whether unipolar and bipolar disorders are distinct, or part of a single continuum, remains a hotly debated topic. The very common co-occurrence of depressive episodes whenever manic episodes are present suggests that they may be related in some way. On the other hand, depression disorders and bipolar disorders are treated with different medications, suggesting that they may be distinct disorders - depression is typically treated with anti-depressants, while bipolar depression may be more effectively treated with mood stabilizers. The disorders have some predictors (genetic, environmental, comorbidities) in common, and some different. It's also entirely possible that both disorders incorrectly encompass several unrelated sub-types.