There is currently no super-simple integrated way to do this with Joplin, although it is on the roadmap https://joplinapp.org/gsoc2020/ideas.html#1-support-for-multiple-profiles
For now you can use the –profile flag to manually specify a database location which can be anywhere (often users will use an encrypted container). This is not an officially supported method though and only supports one open instance of Joplin at a time.