My first thought was why would anyone need a "back up" for an ACOG. Then I remembered that some are battery powered. However, I opened and read and think that the term "back up" was not quite what the OP was thinking about. IMO, LTUME1978 was thinking about a 1x sight to switch to from the 4x ACOG for CQB.
I put a 4x ACOG on my M4 about a year or so ago and spent a little time getting used to and bonding with it. Getting it sighted in after installing it was frustrating, but once done, WOW!
Since putting it on I've also spent quite a bit of time practicing at multiple ranges from 3-200 yards. Mostly from 3-10 then some at 25, 50, 100, and a little at 200. Getting myself familiar with using the Bindon Aiming Concept ability to be able to use this sight at any range effectively and precisely if needed. Therefore, I don't have a need for a 1x CQB sight.