the life span of each chain is 100,000 miles, so both require changing at that mileage.
the cam chain does all the hard work, so this is the one which snaps, sometimes when it breaks it takes out the balancer chain.
it is up to each owner to decide whether they want to change both, I only changed the cam chains on my 2.2's as the mileages are both below 40,000 miles 🙂