Redundant/Interconnected/Separately Derived System Neutral/Grounding

[Nerdx86]

Background:

• We work in an industry (ag) where our customers usually have the local electricians bring in more than 1 electrical service/generator/transformer to a building for redundancy.. (A/B Power, A/B/C Power, etc) (power failure is a immediate threat to life/health)
• Often, the electricians will interconnect the neutrals of all of the services into a common "System", but sometimes they will leave the neutrals independently grounded.
• Often in these cases, the control circuits are derived using some type of failover device (reversing contactor, Form C relay, NO/NC contactor, etc)..
• My understanding is that if the neutrals are not interconnected, then the failover device should also switch the neutrals (and usually does), so the neutral current returns to the correct panel instead of flowing through the common equipment grounds. I understand that this is subject to 404.2 (B) and the exception for simultaneous switching.
• IEEE 142-2007 1.6.6 Multiple power sources (b): "Where sources are in close proximity, or where the system is four wire, the common neutral or ground bus as shown in part a) of Figure 1-24 should be used. In a four-wire system the sources would not be considered as separately derived."
• [FONT=arial, sans-serif][/FONT]We supply control equipment, so we are often in the position of gently explaining to our "ignorant" eyes, things might possibly not be right..

Example:

480V Service A ------ 480 to 208 xfrmer A------
480V Generator A --|

480V Service B ------ 480 to 208 xfrmer B------
480V Generator B --|


My Questions:

1. Is there a NEC compliant way to interconnect the neutrals of separate Transformers to create a single "system" in NEC speak? I can't seem to find anything that readily defines this?? Obviously, this interconnect would have to be sized like an unbalanced neutral.. Is it permissible to consider multiple 4-wire 3ph or 3-wire split-phase transformers with secondary neutrals connected to a properly sized common as a single polyphase system? What code applies here to allow or disallow this practice?
2. My understanding of 200.6 is that if the neutrals are not interconnected, they would need to be either separated in different conduit/raceways or use traced wire colors... Is this the correct understanding? So in the A/B example with a failover switch, there would be three neutral "colors"/stripes A, B, and failover?