Merge two "MindMups" to one
Being able to merge another MindMup as a child or sibling to your own "MindMup" will allow people to work on their own part (sibling or child) of a bigger project, and the project manager can be able to put all "sub-mups" (child and siblings) together to a complete project MindMup.
An alternative to merging MindMup's would be making it possible to assign a "child" or a "sibling" to a collaborator and that the only prt of a bigger project he can edit and develop.

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Chris commented
Yes please, I can get extra credit if I have a mind map of the whole Unit so if I can put my individual lessons all into one I can do that.
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Bill commented
I believe this can be accomplished already - perhaps not in a way that is ideal for you - but...
All my mindmaps are stored on google drive.
I have one "Master Mind Mup" and an increasing number of "project Mind Mups" each node of the "Master Mind Mup" is named the same as each "Project Mind Mup".
I then simply paste the link of each "Project" into the appropriately named node of the "Master Mind Mup".
I then find it easy to always open the "Master Mind Mup" and click on the node for the project I wish to work with.
I have taken this a step further by having large projects linked from the "Master Mind Mup" and the large "Project" node itself has nodes which contain links to the sub-projects of the large project.
I hope this is clear - if not contact me and I can explain further.