Where the invention is of a nature requiring searching in widely dispersed specialised fields, a special search division consisting of two, or possibly three, members may be formed, for example, where the "person skilled in the art" in the technical field of the application consists of more than one person (see G‑VII, 3).
Another case is where there is found to be a lack of unity in subject-matter between different technical fields.
In such cases, the documents found in the different technical fields by the first and by the other member(s) are included in the same search report. The search opinion however is prepared by one member only, if necessary in consultation with the member expert(s) in the other technical field(s).
Source: http://www.epo.org/law-practice/legal-texts/html/guidelines/e/b_i_2_2.htm
Date retrieved: 17 May 2021