Hi @chiara_iannetta ,
two docker containers can see each other the same way two computers can see each other: you can connect them over the network.
However I think the simplest solution would be to make ROOT available in the Hub container itself, either by setting up cvmfs (see e.g. how it is done [here](GitHub - eguiraud/cvmfs_everywhere: Recipe for an ubuntu-based docker container with a working cvmfs client or maybe check CernVM File System) or by creating a new Docker image based on the Jupyther Hub image that also has ROOT installed.
Cheers,
Enrico
EDIT: note that if you just want a jupyter notebook with ROOT available it’s relatively simple to create an image that does that starting from the rootproject/root image. An example is here, it basically comes down to exposing the jupyter server to the host.
EDIT 2: you might also want to check out/follow the progress at Adding ROOT C++ kernel to Jupyter Notebook container