I'm not clear on your question. Can you elaborate? What part of the code are you refering to?
If you shut off circuit #24, how would it get power from the neutral of another circuit?

Its been a long time since I worked with Square-D.  I think you are referring to the attempt at load balancing to achieve minimum neutral current.  In your panel box, you only have one neutral, unless you have gone to some non-standard distribution.  Your circuit breakers shut off only the "hot" side, so the neutral (which is also connected to earth ground at one point in your building) is never interrupted.  
This may be of some help, but you could check with the engineering department at Square-D or one of the companies that make distribution panels.

     I think that you are asking about multiwire branch circuits where 2 or 3 ungrounded conductors having equal voltages between them share a common grounded (neutral) conductor.  There is no code requirement at this time to identify what grounded conductor  is associated with what hot.  There is also not a required for a common disconnect for multiwire circuits except those that serve receptacles on a common yoke in a dwelling unit.  The problem comes when working a multiwire circuit with only one of the ungrounded conductors locked out.  This problem is the voltage that will be on the grounded conductor when you open it.  This is a case where the white wire will bite you and is a safety problem.  One of the reasons there is not a code (NEC) rule is because the code assumes that only "qualified" people work on electrical circuits and that a "qualified" person will know the dangers of working on a multiwire circuit without locking out all of the hots for this circuit.  
   A common trip breaker would prevent this problem, but may cause other problems because all of the hots will be opened when there is a fault on any one of them.  I think the best solution would be a code rule that requires the OCPDs that serve multiwire circuits be identified and that the grounded conductors be identified in the panel as to which hots they are associated with.  I believe there are some proposals for the 2005 code that address this problem.  The code making panel's action on these proposals is not available yet, but you can read the proposals at:
http://www.nfpa.org/nec/TheNEC/ROPsROCs/2003ROP/2003ROP.asp
Don Ganiere
   

  For those that don't understand my question, I am refering to 3 phase applications.  
Understand that this is being posted by a canadian electrician..I'm not sure of American colour codes.  Red, black, and blue are all phases of a three phase system that can be grouped with one neutral but there are no requirements for proper groups of say 1,3&5 to be a specific group sharing the same neutral.
  How does one know which red, black, blue is on a specific neutral if it is need a code requirement?

How does one know which red, black, blue is on a specific neutral if it is need a code requirement?

YOU DON'T
If the system was installed by a qualified electrician, the neutral should be properly marked at the neutral bus in the panel with all 3 numbers.

Get a cheap amprobe and clip it on ALL the wires under the wirenut of the neutral you are working on. If there is a current and your circuit is off, it is a shared neutral and treat it as such.

As for a code requirement, no there should not be one for the way we group circuits but i believe it would not be a bad idea to require all branch circuits be marked with their circuit numbers along with their neutrals in the panel and in all jboxes. Personally I mark them all including the neutrals and i also try to group them 1,3,5 but that only works on new construction.

If you are working with a 3-phase load, then the C/B must be 3-phase and open all phases at the same time.

If what resqcapt19 says is what you are actually working with, then you must open all the single-phase C/Bs associated with that neutral to be de-energized.

An important point: the recommendation by SteveHuck about checking with an amprobe is not safe. Just because there is no current flow, don't assume that there is no voltage. It just means that there is no load connected.

As far as a code requirement, for the NEC, I thought that this type of system was no longer used for branch circuits and individual neutrals were required because of harmonic loads.

Dan,
   Multiwire branch circuits are still in common use.  Not all loads are nonlinear.  In some cases the grounded conductor of a multiwire circuit is inscreased in size over that of the ungrounded conductors for nonlinear loads.  

Steve,
   Your comment about a "qualified electrican" marking the neutrals is not a common practice in my area.  I do it and think that it is a good idea just for the reasons brought up in the original post, but it is not a code rerquirement and therefore many installers do not tag the neutrals.  

Don

Right-o Resqcapt19,
"In some cases the grounded conductor of a multiwire circuit is inscreased in size over that of the ungrounded conductors for nonlinear loads."
This is what we faced when wiring a new server room. SMPS's (in the servers and other computer equipment) are highly non-linear and can lead to substantial heating of the grounded conductor. We used MC with a "Superneutral" wire, where the three phases were 12ga wire and the neutral was  10ga.
Foxfur

FYI -- the D.O.E. does not permit "triplexing" circuits as you described for the very safety reasons already mentioned.