Darting Grass-darts

I saw a pair of tiny butterflies in my garden today and went inside to get my camera hoping they'd still be there when I got back. I needn't have worried because they were flying around and settling on low shrubs in the sunshine. Getting a focus on them was another matter. If one flew so did the other. How do they communicate when they're more than a metre apart?

They're Greenish Grass-darts Ocybadistes walkeri, beautiful little creatures. Note that they hold one pair of wings in a vertical position.