You can specify the same data directory with "-datadir" in a .conf file. See
here.
This does not enable you to share a data-directory. Doing that will just corrupt your data directory if you manage to bypass the startup locks.
Actual storage can be shared by using a file system that support copy on write, or copying a data directory and replacing the block files with hardlinks.
Because of those alternatives I wouldn't spend any time developing or reviewing functionality to improve this further, and would instead favor finishing the pruning support.