I automatically assumed that John and Lisa were boyfriend/girlfriend. Comparing it with me and my wife - of course she should be the one to take the antidote. Although I am not sure that I am intelligent enough to have thought of shoving it down her throat without this prompting (or her disagreeing) though.

On the other hand with some one else, who has the same "right to live" as me (my interpretation of your "similar" - ie same sex, same number of kids etc) a coin toss still has problems. I would prefer that we put the antidote and lookalike into a bag and random draw then drink our selected bottles at the same time so that there was no time between knowing who had "won" and being able to do something aout it.