If I interpret your reply correctly then game play under a Coordinated Infantry Assault order is primarily governed by it's two separate activations.
If so then could you please verify if the following is correct?
A Coordinated Infantry Assault order allows activation of two separate Combat Groups that may Rapid Advance together into close assault against one target, but each Combat Group's movement and close assault is resolved separately and sequentially. The first Combat Group's movement and close assault is resolved normally with any valid Defensive Fire and the post-assault effects immediately applied. Then, the second Combat Group's movement and close assault is resolved normally with any valid Defensive Fire from any surviving defenders.
The reason for my initial confusion is because of 6.8.8 - Coordinated Infantry Assault. "This order permits two infantry platoons or combat groups to combine for a single close assault." Also 6.8.8d - "If both are successfully activated they may then move together but using separate 2D6 movement rolls."
These two rules indicate a single large group comprised of two Combat Groups that move together and enter into a single close assault. Hence my confusion since this could easily mean only one Defensive Fire vs the whole group followed by 6.8.8's "single close assault" with 14.9b in the background saying "Close assaults are fought simultaneously." But then 6.8.8f/14.9c indicate both platoons close assault as separate combat groups, one after the other. That's where I got messed up. If I disregard the "single close assault" and "move together" and, as per your reply, treat each activation separately then all is well!
Therefore, I see the main advantage for the attacker to use a Coordinated Infantry Assault order is that it provides secure knowledge in advance (assuming two successful activations) that there will definitely be two close assaults. If instead the attacker just activates both Combat Groups sequentially using Rapid Advance orders he will not know ahead of time if the second activation will be successful.
Hope I've got it right this time!