From what I have read and understand better now, is that there are several different designs of twin plugs. This is my understanding.
For more reliability, twin combustion plugs must have twin ignition coils but are triggered at the same time. Sometimes one for starting and two for running or manually controlled.
For lower emissions, twin plugs in different cylinders of different thermal sizes are in series using a single coil with the plug tips in series. Thus one conducts from body to tip and the other from tip to body. One is for the exhaust stroke of unspent fuel on an opposite cylinder while the other fires during compression stroke.
For more power or combustion efficiency twin plugs in the same cyl. using a differential coil with series connected plugs Note that when in series, one plug arcs from tip to ground, while the other arcs from ground to tip. But they can no longer be both centered in the cylinder, so I suspect equidistant.
The current is broken in the primary coil with a switch (points or transistor) which creates the large voltage spike and is stepped up to the secondary. It triggers before TDC since ionization has a delay before detonation that varies with amount of fuel in the chamber, this is why timing is before TDC but delayed with loss of vacuum pressure during acceleration as ionization time reduces.
Today, ECU's look at inputs from the throttle position sensor, airflow sensor, coolant sensor, MAP sensor and even the transmission to determine how much timing advance to give each plug.
The gap between the spark plug tip and piston is very small and I once destroyed an old boat engine with a newer prop that ran at slightly higher RPM. It started pinging due to thermal pre-ignition and one day the over-travel in the pre-ignition piston reflex caused the piston to crush the plug at TDC and made a crater in the cylinder wall from the debris.