Apart from flyback transformers or MOTs, automotive ignition coils are also used as high voltage sources by many enthusiasts. Especially those classic cylinder-shaped ones, used in carbureted engines pre-1990, since these were driven directly from the battery (through contact breakers) to generate sparks and a sturdy primary/secondary winding with appropriate insulation had to be met.
7.5cm sparks (50 kV)
Now, whether you have the old one or today’s modern coils, the principle stays the same. Any ignition coil is basically a special iron-cored transformer with an open magnetic loop and a high sec/pri turns ratio, with its HV return pin permanently connected to the primary. This setup is then submerged in oil, or dipped in any similar insulation material, like asphalt or concrete – and hermetically sealed.
Most ignition coils as of today are way smaller, since these are driven through your car’s electronic ignition system. For experimentation however, I recommend the former type.
3.5cm arc-like sparks (30 kV)
My approach how to drive these two (came from a SKODA 120) was in anti-parallel through a mains sine-wave chopper, like a light dimmer, similar to my triac phase regulator circuit. To chop the waveform I’ve chosen a thyristor (SCR) instead of the TRIAC, as we don’t need full-wave control, it’d introduce more stress on the coils.
Since the high voltage return output is always connected to the primary, driving two coils this way presents a drawback: the whole circuit, including HV outputs, is on mains potential. The only way how to get HV against ground from this setup is to drive only one coil, making sure that your second primary connection (the one which is connected to HV secondary) is always earthed.
And this is where I start to like my two-conductor TN-C wiring 
For those unlucky ones with a TN-S (separated neutral and PE), or even with ground fault circuit interrupters, this might not be possible at all without using an external insulation 1:1 mains transformer, since you cannot draw current between live and PE.. But what high voltage experimenter has a GFCI installed in? It would drive him nuts.
Schematic, click to enlarge
Simple, isn’t it? Mains with light dimmer in series, potentiometer regulates SCR’s conduction angle; two run capacitors (for current limiting) and two ignition coils in out of phase. I’d recommend lower capacities if you do have those modern coils, though. And because it runs straight off mains frequency, the sparks have a loud distinct growl – totally incomparable to my flyback drivers. It overloads my camera’s microphone, too.
Because a two-coil setup, such as this one, greatly exceeds the output voltage ranges your ignition coils were designed to operate in, you might be plagued with arc-overs to the primary. Don’t let it burn a conductive trace once – the sparks alone aren’t as powerful as flyback arcs, nevertheless use any good insulating material such as glass, silicone, hot glue or rubber to see if it helps.