I think it has everything to do with the engineers experience and qualifications. If the person who performs the test and inspection has completed a specific level 3 qualification in testing and inspection then the industry allows for them to sign both parts of the EICR. I believe some software providers actually give the option to have one field as opposed to the QS field.
If you look at the two main competent person schemes you’ll notice that they both operate differently in this arena. The NICEIC promote the QS model as this is what their business is centred around, competent companies as opposed to individual competence of engineers. Its not unheard of to have a QS in an NICEIC contractor to fill both fields in.
NAPIT on the other hand look at this in a slightly different way. They promote the individual competence route meaning that all electricians registered with them must meet the same standards as the QS needs to meet with NICEIC. This means anyone registered with NAPIT has shown a level of skill and experience that says they are capable of reviewing the results on a document and signing them without a second look.
So to answer your question, there is no requirement to have two signatures. Its employer and contractor preference.
Ive just sent over the electrical specification I wrote which should give you some guidance on what I demand from a competent person.