[noun] + かたがた
AかたがたB means you do B and use it to do A as well: a visit that is also a thank-you, a letter that is also a report. It follows a noun, usually a formal one such as おれいお礼, ごあいさつご挨拶, ごほうこく報告 or おみまいお見舞い, and B is usually a visit or a letter.
かたがた takes a noun, never a verb.
かたがた belongs to business letters, formal greetings and humble speech, next to verbs like うかがう伺う and さしあげる差し上げる. Both purposes are meant from the start; neither is an afterthought. With an everyday noun like さんぽ散歩 it sounds stiff, and for an errand on the way you say がてら or ついでに.
A formal letter often closes by joining two nouns with かたがた: thanks and greetings sent together.