I also have to question the dual ECMs...

You're just setting yourself up for a world of hair pulling nightmares trying to run two ECMs on a single engine. As Delcowizzid was saying there's an HEI module that already does everything you need in sending puleses at corrected times to the ECM, while providing the oddfile spark.

Another, better option than going dual ECM is to go to an aftermarket ECM that WILL run oddfire, such as a Megasquirt. You'll spend less money and time in the end just going with something that will be known to work than trying to make some frankenstein of a mishmash of parts to get something that will likely only work marginally at best.

Like has been said, the closed loop fueling will be fighting each other. unless each bank is COMPLETELY isolated from one another, and treated as two separate engines, no no chance of fuel that is commanded by one ECM ever being read by the O2 sensor feeding the other ECM.

Regarding the IAC, yes, one ECM will control it, and you only want one ECM to control it, however, the ECM that has no IAC connected will not like that very much when it commands an idle speed change and doesn't see it. Keep in mind that just because both will be the same ECM and will have the same BIN file does not mean that they will be in sync, that would take communication between them, which is not possible, since they were not designed with that in mind.

The only successful dual ECM applications I have seen are where each bank or sets of cylinders have been completely isolated from each other in both fuel delivery AND spark, where each section was connected as it's own independent engine that just happens to share a common crank with another engine. Trying to split the ignition control like this will just be a night mare.