Multiple-component products needing SDSs
Some hazardous materials are sold in two-packs or three-packs for mixing together by the end user, who would often be a professional user, and therefore require an SDS). The classic example would be a two-part epoxy resin glue.
In this situation, an SDS is required for all individual components (if they are all hazardous), so you would need two SDSs, one for each component of the epoxy resin glue from our example. If one of the components is hazardous and the other is not (and does not contain any materials which might require you to issue an SDS), you only need an SDS for the hazardous component.
In some circumstances, mixing multiple parts together creates further, new hazards, which are often transient. These new hazards can be handled by including warnings on the label and/or instructions for use, (e.g. “may form flammable vapours during curing”, or similar). The hazards could also be included on all of the component SDSs, possibly under section 2.3, e.g. “may form flammable vapours when mixed with substance x during the curing process”.
Producing an SDS for a mixture created by a user is voluntary, and from a practical level only really makes sense if the hazards are permanent rather than transient, and cannot be handled on the label or instructions, or the component SDSs.