Take a sheet of paper and fold it once. Two things happen in that single movement. The paper is now twice as thick, and it now has two layers. Twofold stands on exactly that fold: look at how much, and it means double; look at how it is made, and it means two parts. The -fold ending is doing the same work in and tenfold.
From the word’s history
The source in this packet gives twofold from Middle English twofold and Old English twēofeald, equivalent to two plus -fold. The fold in the spelling is the fold in the picture.
這份資料寫得很清楚：twofold 來自中古英語 twofold、古英語 twēofeald，等於 two 加上 -fold。字面上的「摺」跟畫面裡的那一摺是同一個。
Wiktionary entry for twofold supplied in this packet (kaikki:3f6f69a190036ae4:671778101) — from Middle English twofold, Old English twēofeald, two + -fold
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