Data from a bonehead with a voltmeter. Distributor update.
 

Here's a recap:
993 motor in '83 SC

Running (badly) on one distributor.

Coil wire #1 will give spark to either distributor #1 or #2

Coil wire #2 will NOT give spark to distributor #1 or #2

Voltmeter shows top of BOTH coils conducting at 10 Volts.

Aha! A bad coil wire? No. The ends of both coil wires conduct at 10 Volts.

I understand very little about these motors and even less about their electrical systems. Any ideas?

Moses,

Too bad I don't have a 993 shop manual. In general you should be able to swap parts over and isolate what part causes the failure.

It seems you ruled out everything from the distributor downstream. That leaves the following components. There are two of each:

- wire #4 (going from the center of each distributor to a coil)
- coil ignition coil
- wiring to the ignition coil (ground)
- ignitor (semiconductor device firing the coil)
- wiring from the ignitor to the DME harness
- DME harness
- DME