Thorspark uses a single trigger and a coil with with two output plug leads, a so called "double ended coil" This fires both cylinders at the same time, as outlined by Bill and Ewen. The fact that one side fires consistently indicates the trigger is working fine, the fault must lie in the coil itself, the lead or the plug.
Swapping the leads over....fault moves to the other cylinder, suspect the coil/lead. Fault remains, it's the plug. Time it up on on either cylinder, right hand one side is usually used as this is the side most convenient when setting and checking with the normal magneto.
HT voltages produce strange tricks and without professional test equipment a coil fault will be hard to pinpoint. A word with the makers is worth a call. In the old days it was "test by substitution". This would be expensive. They may offer a test facility if this proves necessary.
Sooty plugs. Suspect choke operation and mixture strength. Bore or inlet guide problems will give oily plugs rather than sooty, together with blue smoke. Rich mixtures give black smoke, poor lumpy running and the smell of fuel in the exhaust.
Swarfy.
