In DITA 1.2 you have two options:
1. Use conref push to push different content into the same base topic used in two different locations. Use @keys on the topicrefs to the topic being pushed to and use @copy-to on each topicref to ensure you get distinct result files.
2. Create a base topic that has the common content and then create separate topics that have just the differences and conref the common content into the topics with the differences.
As Radu says, DITA 1.3 key scopes address this requirement much better, if DITA 1.3 is an option for you.
Cheers,
Eliot