Multiple PostgreSQL instances for one PGDATA folder
No, that is impossible. PostgreSQL protects against that as good as it can, but if you manage to start a second postmaster, instant data corruption will ensue.

This question makes me wonder why you want that:


It cannot be scalability, because you have I/O as a bottleneck.
It cannot be high availability, because the shared storage is a single point of failure.


Such architectures like Oracle RAC are ill-conceived and mostly useless; nobody would design such a system these days.

The way to go is a shared-nothing cluster like patroni.