An odd coincidence-- my spouse has just been reading aloud (for the second time) the Terry Pratchett Discworld novel "Carpe Jugulum"-- a spoof (among other things) on vampire lore. Since in it the vampires do magpies instead of bats, he uses the old rhyme to make commentary. The version he uses is the "two is mirth" one quoted above, and since he's set the tale in a setting that takes off on rural Britain, I'm thinking that's where he borrowed the rhyme from as well. Any Pratchett fans out there who might know? Being a Yank I know I miss some of his references...