Note that un-grounding of the coil's primary winding is the event that's supposed to send high tension voltage throughout the secondary loop.
While the coil's primary winding is grounded through the points or electronic ignition,
the coil
charges. For example, while the points are closed.
When the coil's primary winding becomes un-grounded (for example, when the points open), the coil fires.
Both spark plugs are supposed to always fire simultaneously.
Failure (for whatever reason) of the primary winding to be successively grounded and un-grounded by the points or electronic ignition results in no coil firing throughout the secondary circuit.
Any disruption in the secondary loop (for whatever reason, such as bad plug wire, bad plug, bad plug cap, bad connection between plug wire and plug cap, bad connection between plug cap and plug, bad connection between plug wire and coil, etc.) results in no coil firing throughout the secondary circuit.
Here's an example of a disruption in the secondary loop:
Good Fortune!