You misunderstand, sionnach, and not for the first time. A particularly ruthless individual may well swoop felly twice, thrice or any number of times, on different targets - but one can't by defintion experience - that is, fall prey to - more than a single such swoop.
I'm really thinking of the head honcho of the Nazgul, the Witch-King of Angmar, on his faithful fell beast here (I think the fell beasts are more or less the fellest swoopers known to man).
Dr. Jamieson provides these colorations of the meaning of fell: adjective - keen, hot, biting, singular, extraordinary. substantive - lot, fate, destiny; a large quantity. --Dr. Jamieson's Scottish Dictionary and Supplement, 1841.
Fell swoop occurred in Santayana's The Life of Reason. Vol ! (1907). It is spelled out in two words, but it is used as one word. Should it not be hyphenated? Fell - to take or cut down and swoop - seize with a sweeping motion. ORIGIN: from Shakespeare's Macbeth ( iv. iii. 219).
I seem to be running across more and more incorrect uses of this phrase lately. Here's one from the Washington Post:
"In just six days, Israeli Defense Forces advanced to the edge of the Suez Canal, and in one foul swoop, gained control of Gaza, the West Bank, the Golan Heights, the Sinai Peninsula, and the whole of Jerusalem."