Basically I normally use Kodak's formula for ECN-2 but (1) use some sodium hexametaphospate (I can tell you how much tomorrow) instead of the Kodak anti-cal, and I up the CD3 to 5.2 mg/L. I use C41 bleach or use the Kodak formula for the ferricyanide bleach. Stop bath - if you really want to use sulfuric acid you can mix from battery acid (from Autozone), but I find that white vinegar mixed 50-50 with water does just fine. C41 fixer seems fine. I'm sure some here will castigate me for this anecdotal advice. Negatives straight processed this way on up-to-date ECN-2 stock (such as Vision2 or Vision3 or Eterna) scan exceptionally well, subject to contrast adjustment on the computer; at least as well as Portra and maybe even better.
I haven't scanned the double-developed negatives yet. They are in my darkroom downtown -- I'll bring them home tomorrow and scan some. If you want to see some scans of regularly processed ECN-2, let me know.
Yes an example of each in similar light situations and similar scene would be great, I know that's not always possible but it's a start