I haven't seen a "standard" fork-crown bracket that allows one to mount two headlights. There are, however, a few possibilities, most of which are described on Peter White's website.

To that, i would add my own comments and suggestions.

1. I have tried a main light at the fork crown and a secondary by the side of the wheel. While most people don't seem to be bothered by the shade of the front wheel, I am. I find it easily camouflages potholes, especially when riding with a single headlight... which happens often because I'm not a speed bullet. So no fork-mounted headlights for me.

2. A headlight too close to the side of the tire gets dirty from rain or snow very quickly. Placing it above the fender solves the problem.

My own setup – and how it evolves over the years – is described on this page. Just make sure that the lights are placed in a way they won't get banged when the bike steers completely (i.e. when parking).