Commingled materials?

My first question is whether you are actually meeting the intent of the credit. You might want to check out the MRc2 CIRs. While I understand that the v2 public CIRs do not apply to your LEED v2009 project, it does provide some insight into what is acceptable to GBCI for compliance. Getting reusable materials back into service seems to be a significant portion of this credit, beyond simply keeping them out of landfills or incineration. Your concrete and gyp are two of those material types. Although there are CIRs that may appear to support the backfill type approach I thought you might want to be aware of at least 2 USGBC rulings below that clearly go the other way so you could plan accordingly:

6/19/2008 - Ruling
The applicant has requested acceptance of mine backfilling as an equivalent means of accomplishing construction waste management. Based on the description of the process, it does not appear that this practice will meet the intent of the credit. In addition to encouraging the diversion of debris from landfills, the credit intent includes an expectation that projects “redirect recyclable recovered resources back to the manufacturing process.” While it is recognized that using clean waste material from construction rather than other potential materials to back fill deep mines may be environmentally preferable, this practice is not aligned with the intent of the credit. The waste is in essence going to a “landfill” and the specific materials noted (gypsum and glass) are known to be potential feed stocks for manufacturing processes. By using them as fill material, the proposed practice is burying potentially valuable raw materials. Underground waste disposal is not a recognized means of recycling for this credit.

7/2/2008 - Ruling
The project team is requesting that on-site burial of concrete from the wash out area be considered landfill diversion.

No, the intent of this credit is to, “Divert construction, demolition and land-clearing debris from disposal in landfills and incinerators. Redirect recyclable recovered resources back to the manufacturing process. Redirect reusable materials to appropriate sites.” The described actions are not much different than creating micro landfills. While there are environmental benefits to on-site disposal in the avoided vehicle miles, the described actions do not reuse or recycle this material.

To answer your original question, no, these do not sound like comingled materials. If your containers of materials are already separated on-site by type and volume or weight are calculated individually for your project, then it is considered diverted and not comingled for entries on the LEED credit form. Your term “mixed concrete” and “mixed gyps” are a little confusing. Since it does not sound like these materials are being sent to the recycling facility anyway, then they would not qualify as comingled since you will not be able to get a letter of certification for the annual recycling rate of the facility, nor are the materials being separated off-site from what I can tell.

You're allowed to divert materials from the landfill in this way, i.e. to use them as fill.