Here's a pretty good photo I found: http://www.minitrains.com.au/images/products/T0M0-g3.jpg
The resolution is good enough to zoom in and look at the two dual gauge turnouts in the background. I must say the dual gauge single slip is rather nice, but definitely not for beginners!
Don't get too worked up over what seem to be conflicting wheel standards. The spacing of the wing rail (beside the V in the crossing) is not important, as that rail is not touched by the wheel flange. It is actually just there to reinforce the end of the closure rail. The important dimension is the checking gauge, which is from the back of the check rail to the toe of the crossing.
When the wheelset meets the check rail, the check rail engages with the back of the flange, and ensures the flange of the wheel on the other side cannot strike the toe of the crossing. So if the flangeway of the crossing is built at 9mm for 7 1/4 gauge, it has no effect on the 5" gauge wheelset - the stock rail prevents the wheel going the other way. The worst you might get is a click if the outside edge of the wheel drops off the closure rail before the tread meets the toe of the crossing. I actually doubt that would happen.
Widening gauge is a good fudge if the check rail is on the common rail, but bear in mind that on real railways they often swap the common rail from one side to the other in order to simplify the geometry of the dual gauge turnout.
There is a lot of geometry involved in building turnouts, but you can get away with a hell of a lot too, as long as your back to back and checking dimensions are correct!
Where are you? I haven't done a dual gauge turnout, and wouldn't mind having a go...