Always try to avoid having two services using the same DB. Only way I'd ever consider sharing a DB is if only one service will ever modify it and all others only read.

Good luck enforcing that :)

The 'collision' is two service classes both trying to use one db.

If you separate them (i.e. microservices) the they no longer try to use one db.

There is nothing stopping multiple microservices from using the same DB, so of course this will happen in practice.

Sometimes it might even be for a good reason.