Two things going at once on the same person, one running underneath while the other is the point.
序 · The move
1Pick the act that stays running underneath — the backdrop — and put it first.Could this act keep going on its own for a while? If it is a one-shot, it is not the 著 verb.
2Hang 著 on that first verb, then add its object.著 sits on verb1, never at the sentence end and never on verb2.
3Put the main act second; the same subject does both at the same time.Read verb1著 as how/while, verb2 as the thing chiefly done. Flip them and the sense changes.
例 · Examples
1他們吃eat著kept going as a backdrop餅乾cookies看watch (the main act)電視TV。
V著1 has one verb whose result rides on the doer (戴著帽子, the hat stays on). V著2 has two verbs: the first held open as a backdrop, the second the main act done across it. Count the verbs — one means worn result, two means while-doing.
一…就… chains two acts in time, the second the instant the first lands. V1著V2 runs both at the same time, the first propped open under the second. Sequence versus overlap.
✗ 他們吃餅乾著看電視 → ✓ 他們吃著餅乾看電視
✗ 他們吃餅乾看著電視 (for: eating is the backdrop) → ✓ 他們吃著餅乾看電視
✗ 他們吃著看著電視餅乾 → ✓ 他們吃著餅乾看電視
English ties the two acts with a separate word, while or and, so learners hunt for a conjunction and drop 著 onto the wrong verb or the sentence end; here 著 alone, sitting on the backdrop verb, carries while.