I believe that https://issues.civicrm.org/jira/browse/CRM-19041 will provide you with the answer you are looking for - if you select both as the option under "Contributions OR Soft Credits?" in your search criteria and then export the results using primary fields, you will get the soft credit information listed on the same line as the contribution. The odd thing that I hadn't noticed until now, however, is that contribution ID is not considered a primary field - you can only select it as a field for export. 

Hope this helps!
Tamar

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