thinking about this exact same problem, specifically with reference to her and her boyfriend's shared save file on Super Mario Galaxy.
Although things start well with sharing the game -- switching off every other planet, and dividing play-time equally -- the plan goes a bit haywire when she visits home for Thanksgiving, and her beau just keeps on playing... and beats the game without her.
Of course, we think the answer is obvious: separate save files for separate people. Single-player games are hard to turn into shared experiences, especially when both parties desire equal play-time. Read about Katie's upsetting experience on gaming with a significant other, and share your own success/horror stories below.