A four-character idiom meaning to achieve two gains with one action; similar in casual use to 'kill two birds with one stone'.
図書館で勉強すれば静かに読めて集中できるし、交通費も節約できて一挙両得だ。
Studying at the library lets you read quietly and concentrate, and you also save on transportation — killing two birds with one stone.
出張で得た時間を観光にも使えば、一挙両得になる。
If you use time gained on a business trip for sightseeing too, it's two gains at once.
この方法ならコストを下げつつ品質も保てて一挙両得だ。
With this method you can lower costs while maintaining quality — two benefits at once.