AFAIK in the quench area is useless as JIm describes.
My choice for a second plug would be at the farthest end of the "transfer" area above the bore, just before the quench area.
If both plug positions are new, I would try there, and the second plug at the exhaust valve (rather than the intake).
You can use smaller plug bodies (14mm, 12mm, 10mm) for more choices of location, but as they get smaller their fouling resistance goes down because the path to ground on the porcelain is shorter.
Remember that a second plug reduces spark advance requirements - you can't run as much, I would start with 5 degrees less as a test.
The 2 plugs will almost certainly not run at the same temperature, the plug near the exhaust will be hotter, you may need 2 different ranges (B6 and B7 etc.).