| I think that is the combined grid is both DD classes, then the grid should all start together in qualifying timed order - a la Assen. You'll still get the "fast in a straight line, slow cornering" argument, but I feel this is lessened if DD only bikes. And the riders are typically more respective of each other.
If the grid is combined with another non-DD class then staggered starts are best - the other class are likely to have faster bikes, will have tyres better suited to the conditions (wets) - and are less even with the DD bikes.
Tim | 
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