Google "drilling contractor dual bop" and click on the top article in drilling contractor magazine, which is called: Dual BOPs: When one just isn’t enough.
Makes interesting reading
BOP = blow out preventer.
Having two instead of one more ore less halves down time between drilling of wells
Have you ever seen a seven ram BOP in person? Each of those things is the size of a three story building. Each of the rams should be able to stop the flow of oil, so a seven ram unit already has multiple backup systems. It does make since to have a backup, so you can recondition the first one while the second one is used in the drilling operations.