You can transfer repositories/sources from one tree to another with the following steps:
Choose the wanted repository (lists => repositories => YOUR_REPOSITORY)
Add the chosen repository to the clipping cart (Clipping cart => Add to clipping cart). This will also copy the souces of the repository to the clipping cart.
Repeat these steps for all the repositories you need.
Download the content of the clipping cart (Clipping cart => Download)
Create a new tree in the Control Panel and import the downloaded file
Merge the newly created tree into another tree (Control panel => Family trees => Merge family trees)
With this approach, you can transfer the data one time. However, it is not possible to share the data between the trees (and only maintain in one of them), because webtrees does not support data sharing between trees in general.