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Best practices for local development. Is it ok to share one user for multiple DB's?

Context: Let's say I'm developing a theme for client A, and a theme for client B. I'm developing these themes locally. I've set up two separate folders, each with their own WP install and DB.

For convenience, is it ok to share one user & password for both installs and DB's?

If this is on your local machine then I don't see why not. Once you go to a staging or production server then the details will change and it's best to use different credentials then.