Researchers experiment with individual and group of designers. In engineering design what are pros and cons (in general) for doing experiment with two designers in a group?
Nothing. There is no statistical significance, but a high level of subjectivity instead.
Collaborative design is thought to have some general advantages and disadvantages. Depending on your experimental setting, the collaborative aspect could improve the outcome for each group. Importantly, the individual designers may be encouraged to express their ideas/understanding (sketches, notes, conversations) when working together. This might help the group keep track of the design process, as opposed to individual designers who tend to make fewer snapshots of their process. So if the experiment is comparing the process or the outcomes, grouping participants can be helpful. A benefit of having individual participants is the number of data points you collect.