Using one system all of the time and the other for back up is one way of helping to ensure at least one system is hopefully always fully serviceable. The same logic can be applied to other redundant systems where only one is in 'use' at any one time.
If both systems are used equally, on alternate days for instance, you are more likely to have failures around the same time as the associated components have had the same amount of use. Obviously, it is still rare to have dual failures and there are other factors involved other than hours of use but it made sense to me when a colleague explained it to me! That said as an aircraft accumulates time, more and more high failure rate items are being replaced introducing splits in individual operation times regardless... ah... do whatever the SOP says
