In this scenario it seems that the Mainline Stream twin of a.txt is the most important and its history should be easier to get at in the future than the other twin from Feature Stream.

You could try the following steps to resolve the conflict and avoid constant conflicts as changes are made to a.txt from the Mainline Stream:

1) Copy all of the latest contents of a.txt from Feature Stream to the clipboard

2) In Private Workspace, discard A and B plus any "Merge" change sets you have involving a.txt

You can do this from the pending changes view. Right-click on the component with a.txt in it and choose "Show History." It will provide a list of all change sets accepted into the component in your repository workspace. Multi-select the change sets, right-click and choose "Discard..."

3) Accept change sets A and B again

4) Resolve the conflict with "Mine"

5) Paste back in the contents of a.txt copied from step 1

6) Check-in the change of contents into the "Merge" change set created in step 4

7) Deliver change sets A, B and Merge into Feature Stream

The users working on Feature Stream will now be working with the a.txt twin from the Mainline stream. The history of the twin of a.txt in the Feature Stream is still preserved and can be discovered through the Merge change set because it contains a change to both a.txt twins. When Feature Stream eventually delivers everything to Mainline Stream all of the change sets and history of both twins will flow over.