At the moment it's not possible to synchronize multiple fields to one field as this might create conflict. However, if you are working with different issue types, no conflict should be given.
Steps to reproduce:
Go to the field mapping tab
Set up two field mappings:
Text field A (applied to only bugs) -> Description
Text field B (applied to only tasks) -> Description
You should be able to set this up as there is no overlap for the description field
Backbone throws an error as you can't have more than one field linked to the same field.