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

I believe https://issues.civicrm.org/jira/browse/CRM-17286?focusedCommentId=80228&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-80228 will provide you with the answer as to why there are now two lines in the standard contribution export. That said, a couple of things that you can do to get around the issue:

1. Use the "Contribution Detail" report and include the column for "Contribution OR Tribute?" as well as any additional columns you need - when you export to CSV, you can filter/sort on the "Contribution OR Tribute?" column with the understanding that for every soft credit listed in the report, there will be an additional line for the "hard" contribution - depending on what you use the soft credit information for, you may want to save an instance of the report that will list only contributions and another that will list only tributes and then export each separately.
2. If the first option doesn't allow you to display all of the information you need, and if the the number of contributions with soft credits that you deal with is relatively few on each import, after exporting to CSV, CTRL + down arrow in one of the soft credit columns to arrive at the first soft credit in the export, then delete the corresponding contribution line that does NOT contain the soft credit information (from what I can tell, it is typically the line above the one with the soft credit).

Hope this helps!