を兼ねて|をかねて

Indicates that one action or event also serves as another, so that two purposes or functions are combined in a single occasion: 'also serving as', 'doubling as', 'combined with'.

Used in both speech and writing, and common in announcements, business contexts, and explanations of why something is done. It presents one occasion that fulfills two purposes or functions at once. Compared with the formal, set-phrase かたがた, it is more neutral. Compared with がてら or ついでに, it focuses on a combined purpose rather than an extra action added along the way.

• 1.
  がてらN1「がてら」 attaches to action nouns and can also attach to the ます-stem of a verb; it is mostly used to stop by somewhere while out walking, shopping, or the like, and it is also used in relatively formal expressions such as 「ご挨拶がてら伺います」. 「を兼ねて」 attaches only to nouns, can take the form 「を兼ねた＋noun」, and is not limited to outings, so the two differ in emphasis and cannot be broadly interchanged.
• 2.
  かたがたN1「かたがた」 attaches to action nouns without 「を」, typically nouns like 「お礼」 or 「ご挨拶」, and it has a formal tone, appearing mostly in visits, greetings, and letters, though it is also used with things like walks. 「を兼ねて」 is neutral in register and also has the 「を兼ねた＋noun」 form; in formal situations the two can often be interchanged.
• 3.
  ついでにN3「ついでに」 means doing something secondary while taking the opportunity of doing something primary, with the main and secondary actions clearly distinct, whereas 「を兼ねて」 means that one thing itself serves two purposes. 「ついでに」 can attach to verbs or nouns, or stand alone as an adverb, while 「を兼ねて」 attaches only to nouns.
• 4.
  かたわらN1「かたわら」 refers to doing something else long-term in addition to one's main occupation, while 「を兼ねて」 means that one thing serves two purposes and can describe a one-time action. 「かたわら」 attaches to the dictionary form of a verb or to 「noun＋の」 and leans toward written style; the two cannot be interchanged.
• 5.
  をきっかけにN2「をきっかけに」 means that the preceding event becomes a trigger leading to later change or a new action, so the two parts have a clear order and a cause-and-effect relationship; it does not express two purposes at once. 「を兼ねて」, by contrast, means that a single thing simultaneously serves two purposes, with no before-or-after, so the two cannot be interchanged.

かたがた is closest, since it also expresses one act serving two purposes, but it is formal and largely fixed with nouns such as お礼 or ご挨拶 before verbs of visiting or writing. がてら attaches mainly to actions such as 散歩 and adds a second action done along the way, and the more everyday ついでに means taking the opportunity to do something incidental. かたわら describes an ongoing parallel activity beside one's main occupation, while をきっかけに marks a trigger that starts something, not a combined purpose.