Pair programming is increasingly attracting researchers' and practitioners? attention. One of the claimed benefits of pair programming consists of easing socialization among programmers, with the effect of transferring tacit knowledge. Designing software systems requires a strong employment of tacit knowledge, such as individual experience and skills. In this paper We explore the hypothesis that working in pairs can speed up and enforce the knowledge building process among designers. We name "pair designing" the application of pair programming concepts to the design stage. An experiment has been performed to test the effect of pair designing on knowledge building. This paper discusses preliminary results, which confirm the hypothesis of a positive effect of working in pairs on the process of knowledge building.