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i think hot rod magazine did an article on charging agm batteries a couple of months ago. iirc it said something about hooking a flooded battery up beyond the agm to trip & trick the agm to take a charge. i skimmed over the article so i may have missed something lol.



That technique is for charging a deeply discharged Optima with a conventional charger. You're not tricking the battery but rather the charger into giving a charge.
A deeply discharged Optima has very high internal resistance, some 'ordinary" chargers see this (or more precisely DON'T see it) and fail to provide current. The parallel battery serves as a load so that the charger works, once the Optima is no longer deeply discharged (and its internal resistance drops) it can be removed from the circuit and the charger will charge the Optima.