double whammy usage for two good things?

Solution 1:

A whammy is never good.

Origin 1940s: from the noun wham + -y1; associated from the 1950s with the cartoon strip Li'l Abner, in which the hillbilly Evil-Eye Fleegle could ‘shoot a whammy’ (to put a curse on somebody) by pointing a finger with one eye open, and a ‘double whammy’ with both eyes open.

The antonym to a curse would be a blessing.

So you could use double blessing

Solution 2:

In addition to mplungjan's double blessing (e.g., The parents were doubly blessed in having twins), one could also say that sbdy had 'twice the luck' or was 'doubly favoured.'

But a more exact opposite to 'whammy' would be 'stroke of luck' or 'lucky break' or maybe even 'fluke' in the right context. So the opposite of 'double whammy' would be something like 'two strokes of luck' or 'a pair of lucky breaks.'