On the surface, this seems to be an easily answered question.
After all, we're issuing an overall score for the entire product, and that product often includes both single-player and multiplayer entertainment. Hence, both must be considered, and it's up to the critic's discretion as to how heavily to weight each side; for instance, the multiplayer might be more heavily weighted in Medal of Honor: Warfighter than it was in Resident Evil 6 .
On top of which, there's a general assumption that the majority of those who buy a game will play both the campaign and multiplayer. However, as time goes on, I keep running across people that almost exclusively buy titles for one reason or the other, and I've also heard of people only reading the parts of reviews that deal with either the single-player or the multiplayer. Furthermore, it really is true that the two experiences are extraordinarily different; quite separate and distinct beasts, in fact. Therefore, maybe it's not such a bad idea to split the two up and deliver two scores for certain games.
Some may say this could pose problems because readers might get confused, but it's not like we have a standardized form of reviewing games. The classic is still a 10-point scale but that's hardly the only system used by major sources, and some – like Game Informer – hand out multiple scores (from different critics). So it's not like we'd be breaking some sort of cardinal rule and besides, it could really help consumers make easier purchase decisions. It also doesn't help that reviewers approach the analysis process very differently; some spend a lot more time talking about multiplayer (or the campaign) than others.
What do you think?