[logichub]

Is it possible to install bbPress on a separate WordPress either in sub-domain or sub-directory of the main domain provided that both sites can be accessible with a single login?

[Stephen Edgar]

Yes, you sure can, either way you mention would work, another option is multisite.

That said, for either way you mention you’d utilise “shared user tables”

See this codex doc on setting that up: https://codex.wordpress.org/Editing_wp-config.php#Custom_User_and_Usermeta_Tables