I've never seen a multipart rar file with different or multiple passwords. Also, when you open a multipart rar and it asks you for the password, you just have enter it once and it will automatically pull the parts from all of the "linked" parts (in addition to checking that they're all there). That's how they work. If they're working some other way, then they're not part of the same series.
If you're opening these two separately with two passwords, the files contained within them are not part of a whole and the files were likely packed separately.
Best bet, as Jason2074 suggested, is to inquire with whoever provided the files.