In my above post, I have assumed that the engine still has the original ignition system. If it has been modified by fitting a 'modern?' twin spark coil, the HT side of the distributor will have been bypassed, and the engine will be running a 'wasted spark' system where both cylinders fire together at each TDC, so it doesn't matter which lead goes to which cylinder. With this system, it is still necessary to keep the distributor cap and rotor arm in place to prevent the points cam lifting off it's driving pins. Also, when testing for a spark with this system, it is necessary to have the whole system in place in order to get any sparks. With 'twin spark' coils there is no internal connection to earth for the secondary winding, so the HT circuit is - cylinder head, spark plug, HT lead, coil, 2nd HT lead, 2nd spark plug, other head. Leaving one HT lead dangling will result in no sparks at either lead.
Regards,
Eddie.
P.S. Whilst tck makes valid points, the Miller ignition system fitted to Dragonflies is not the most robust, and therefore subject to wear, making the fully advanced timing more important than the 'static' timing - hence, my previous comment regarding fine tuning once the bike is mobile.