What does it mean when an aircraft engine is de-paired?
De-pairing means literally braking up pairs. In the case in question, it means that if an aircraft is equipped with engines of a certain specific type, one of the engines has to be replaced with a version that has been upgraded.

The reason for the de-pairing is to mitigate the risk of a dual engine failure: Having two engines that are both affected by the same issue considerably increases the risk of both engines failing on the same flight. At least RR Trent 1000 and P&amp;W 1100G have been issued a de-pairing AD by EASA.

Article on RR Trent 1000 de-pairing AD on Airlineratings.com

Article on P&amp;W 1100G de-pairing AD on samchui.com