The past few days as we added new individuals to FamilySearch.org's Family Tree directly from Ancestry.com's family tree, we noticed that the resulting individual in Family Tree had all of the sources from Ancestry.com. The same thing happened when we added a new individual to Ancestry.com's family tree from Family Tree. i see this as a big step towards the ability to fully share sources between the two online family trees.
Un fortunately, I cannot simply demonstrate the process without having an individual to use as an example and all the examples I saw this week were from other people's family trees. None-the-less, here is a screenshot showing the link in Ancestry.com where you can add a family member to the family tree:
Clicking on this link will bring up a list of the individual's family members which can be added one by one. My family tree on Ancestry.com has been there for years and I don't have any particular individuals I need to add from the FamilySearch Family Tree. But if you do have someone you wish to add, you can do so by clicking on the link and selecting an individual to add.
The ability to share data between the three large genealogy database programs and Family Tree is in the development stage and you can expect developments to happen unannounced. I am guessing, but I would think that this will be a feature highlighted at #RootsTech 2015.