At Harvey Mudd College we call solo-programming "Buddy Programming" because students are expected to continue to engage with their partner as they work. Sometimes pair programming can help establish this collaborative relationship. Tell students exactly how long they will be pair programming so you don’t have students ask "When do we get to work by ourselves." They might not realize how this comment might make their partner feel.
Example: "In 30 minutes we'll switch to buddy programming."
See this and related tips as a Tip Sheet: http://csteachingtips.org/tips-for-pair-programming