Post G-OBMM, the recommendation was NOT to perform critical maintenance on both engines of twins at the same time.
I don't think that recommendation (which was only to "consider issuing advice to aircraft maintenance organisations that, where practical, work which can effect [sic] the airworthiness of an engine should not be conducted on all of the powerplant installations of an aircraft at one point in time by the same personnel") ever made it into the rule book, except inasmuch as there are, I believe, corresponding provisions which apply specifically to ETOPS.