Caz, I'm not familiar with the altimeter setting SOPs which applied to the Chinook force at the time, but can you be certain that with 2 baro alts and (I assume) a rad alt, that there was any mandatory requirement for both pilots to have the same subscale setting at that phase of flight?
Various aircraft have different altimeter SOPs. For example some trucky aircraft with 3 altimeters often used to fly with a different subscale setting on the back up altimeter until top of climb. They also had very different rad alt SOPs compared to other aircraft I've flown.
How different were the subscale settings, by the way?