When a student asks a question, make sure you address your answer to both of the students and take time to check that both students understand. If one student understands and the other student doesn’t, stay with the pair while one student explains it to the other one. This shows that explaining the idea is a learning opportunity and not to save you time.
Example: "Have you and your partner discussed your question?"
See this and related tips as a Tip Sheet: http://csteachingtips.org/tips-for-pair-programming