Thank you for your comment
MultiEditor allows to work with attributes inside one product - there is no way to use values from different product at Expressions. You can copy description from your main product, select few products that need to be updated and simply paste data at MultiEditor:
http://screencast.com/t/mSVnjpFpCv8
Also, you can export descriptions to the file, update your file (add same description and short description to the file) and import the file back to your database
Other way to update products based on values from another product is to create SQL statement (here you can specify what values should be used to update based on id).
Waiting to hear from you