The problem is database locking.  Bitcoin Core assumes it has complete control of its data directory.  So sharing a directory with multiple instances of Bitcoin Core would quickly result in data corruption.  The best you can do is periodically checkpoint the database directory (be sure to first stop the node using the directory) and then use the checkpoint to initialize the database directory for a new instance of Bitcoin Core.  I have done this several times to reduce the time it takes to create a new instance on a different system.