I have a Apple phone 6S Plus and a Apple Ipad should they both have the same apple id and password?

Yes, if they are using the same Apple ID, but if you are referring to a "Passcode," those may/might be different.

That depends on how you want to use the 2 devices. If you want them both to have the same data synced to iCloud, the ability to download the same apps, books, and other media without paying for them again, the same messages in Messages, and any other shared data, they need to use the same Apple ID and password.

If you want them to have separate data, then you need to use two different Apple IDs and passwords.