There must be some easy way. I was just watching a tutorial a couple days ago where the guy does it several times but all he says is "so, I'll duplicate this mask, flip it and move it to the other side". lol
I just like to try and pick up on those little time saving tips.
I've copied the Mask path and pasted it to an Adobe Illustrator document, used the Reflect Tool in AI, then copied and pasted that result back to another Mask path on the same layer.
I've also made a duplicate of the current layer with the mask, flopped the duplicate (set the Y scale value to –100), then set that as the Track Matte layer of a duplicate of the original layer.