2HDD

Pro : More storage, 2 independent storage devices so you can backup 1 to the other

Con: Not sure there really is one, expense of the second drive? If it's an alternative to buying 1 larger drive there are a few I can think of but there is not direct negative to adding a second drive
 
Pros. Reliable back up for the another disc.
secured data on separate disc when 1 failed..
more storage

cons. need more space
need additional cooling system.
need reliable power supply