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In quite a few of our systems a twin outlet ignition coil is used. Those coils do have a few particularities which you should know
In  coils with one exit:

one end of the secondary coil is connected to the spark plug and the  other to ground.
  In our twin outlet coils:

both ends of the secondary go to spark plugs

typical resistance between both exits is 6.2kOhm

This leads to a few important conclusions:

You have to connect both plugs to make it work!
Ignition will only work correctly if both plug terminals are connected. You may not test one side with the other open (not sitting on the mounted spark plug). This is because (effectively) each exit uses ground from the other. That means also that both plugs are working in serial, adding resistances, so better use low resistance spark plug (resistor) sockets and make sure they are good (if in doubt, measure resistance on a hot socket (warm it up before measuring).

Is the flow from ground one side vis spark plug there, via coil, to other spark plug and its ground interrupted you get no spark - on no side. If you really want to test only one side, put the HT wire of the other to ground (earth it) than it will work.

Sometimes a coil deprived of its ground from the other side searched for a substitute - with a solid fireworks around it to chassis ground.
Resistor sockets, high tension wires and spark plugs have to be good on both sides!
The ignition current has to pass both sides. If components there are bad, those negative values double, ignition gets weaker. If one side fails, both will fail.

Please do not get fooled by good optics of spark plug sockets, many a socket has a corroded resistor coil inside, even if never used. Check resistance with ohm meter, preferably on a hot socket (resistance increases in hot condition). Never use sockets with more than 5000 ohms, better 1000.
Most problems customers report, have been found to be due to bad high tension circuits (ht wires, sockets, sparkplugs). This is even more true with the twin outlet coils. (Small side remark: most of the remaining problems are due to bad earth connections.)
Last thing to mention, both exits always fire at the same time (something many original systems equally do). Different is only sense of polarisation, something that you might notice when using a stroboscopic light.
It is possible to replace the twin coil by 2 single coils if this is really needed for an application.