A "double whammy" is, apparently, a situation where two bad things happen at the same time. What's the phrase where two good things happen at the same time?
'Whammy' started life in 1940s America meaning a hex or 'evil eye' influence, presumably on the basis that a 'wham' was a heavy blow. There, people referred to "putting the whammy" on someone. A 'double whammy' meant putting the power of both one's evil eyes on the victim.
If it can now mean good things as well as bad, that's a recent development.