Ah, I never use Export/Import. So, when I saw the option was in the Library drop down in my install of Portable I assumed it did something. I just tried it, and got the same message, sorry about that.
As you've discovered you can't run two instances of calibre (32bit, 64bit or portable) concurrently in the same user account.
The calibre-spy plugin will allow you to open other libraries, it is Read Only for the library metadata - i.e. there is no Edit Metadata functionality, but you can edit the format files, e.g. EPUBs with Sigil, DOCX with Word etc. Or read a book
BR