〜かたがた
while also, partly to (formal)
Noun (often a する-verb noun) + かたがた
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かたがた means doing one thing with a second purpose at the same time, and it's formal: お礼かたがた、ご挨拶に伺いました, "I've come to say hello, and also to thank you".
It follows a noun, especially one of a small set: お礼, ご挨拶, ご報告, お見舞い, 散歩, 出張. It's most at home in business visits and letters.
がてら (N2) means the same but is more casual and often about walks and errands. ついでに is the everyday "while you're at it".
Don't confuse it with かたわら, which is a long-term side activity alongside a main job. かたがた is a single outing or letter serving two purposes.
Examples
お礼かたがた、ご挨拶に伺いました。
I've come to say hello, and also to thank you.
散歩かたがた、パンを買いに行った。
I went for a walk and picked up some bread while I was at it.
近くまで来たので、ご報告かたがたお邪魔しました。
I was in the area, so I dropped by, partly to report back.
出張かたがた、実家に寄った。
I was on a business trip, so I stopped by my parents' place too.
略儀ながら、書中にてお礼かたがたお知らせ申し上げます。
Please forgive this brief letter, which is to inform you and to thank you.