If you are referring to the Complaints procedure being separate from the Nonconformance and corrective action procedure, yes of course it can be separate. Laboratories generally combine documented procedures where possible to reduce the number of documents, however it is your choice; as long as it is effective and efficient to do it that way.
ISO 17025 has a requirement that you have a description of the complaint handling process that can be given to a complainant or other interested party. This could be a basic process flow diagram or a simple, clear step by step explanation. Because there is a need for this, you could as an option include any other information; which you want to keep for inhouse knowledge only; either in the quality manual section on complaints, or in a combined Complaints, Noncorformance and corrective action procedure.