one-two punch

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A combination of two blows delivered in rapid succession in boxing, especially a left lead followed by a right cross.
• noun Informal An especially forceful or effective combination or sequence of two things.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A combination of two punches, one from each hand, thrown in rapid succession
• noun by extension A rapid sequence of two things that has a quick effect