Yes, you are correct, not a good idea to run two resistors.  You can change the plug cap to a non-resistor type. 

RT325, that is the "electro tester" I was thinking of.  The gap is large, but it's not in the presence of a pressurized vapor of air/fuel/oil.  My understanding is the compression pressure makes a big difference.  Bottom line is the plug gap has to be like 100 times smaller to work inside an engine...  like 0.020" to 0.024". 

When I was a kid they used to have spark plug testers that you'd thread a spark plug into, pressurize the chamber, switch on the spark generator, and then observe the spark through a clear window in the pressurized chamber.  That's one reason why I've said many times that a plug can spark just fine laying on the head outside the engine, but fail to spark under compression.  I've seen it. 

Pictures of an old Champion tester that used pressure:
www.philosopherswrench.com/2018/03/champ...ter-restoration.html

About halfway down this page they talk about testing a spark plug under pressure... note that they explain the dangers testing with too large a gap and/or too much pressure...  those conditions can damage internal insulation which is why you should never kick over the engine with ignition on and no place for the spark to go...  plug should be attached to cap and grounded or plug wire should be grounded. 
www.themagnetoguys.co.uk/Spark-Test-Gaps

The original coil expired on my 360 a few years ago, I believe due to my forgetting that the plug wire was left disconnected. I hopped on and started kicking with no path to ground. After re-connecting, the bike ran just long enough to get me down the hill from my house.

I could get spark from the plug wire end to ground, with the cap removed. Bike wouldn't run though. I substituted a later model YZ80 coil that actually fit pretty well and it worked perfectly.

I think the big difference in coils is between inductive and CDI ignitions. As long as you get a coil for a comparable CDI system you should be good. They fail so rarely that a used one is a very solid option.

CDI coils are low resistance on primary
They do not build up a mag field like an inductance coil
CDI's transfer 400 or so volt pulses to the primary side and this change in voltage makes spark on the secondary

Inductance coils need time (dwell) to "charge" mag field, then when power is off, points open, lets this go both to prim. and sec. side.

Condenser buffers points from taking the arc, from the back EMF

CDI magneto and the Yamaha "points" ET magneto coils are virtually identical and, right or wrong, I have used them interchangeably. 

Becoming an oil thread lol. Good though & I'm learning all the time. Keep it coming.
When & if I found a definite faulty coil it would just give a piddly thin spark almost adjusted down to zero on the adjustable gap.
Thanks Mark & others for informative replies.