Due respect, Leif, but I know my AD&D-fu was not so weak. [and PLEASE understand, I am not a "rules lawyer" by any stretch. If you want, as DM, to just make Backstab "x2" and that's all, that's fine with me...as I said, who knows if we'll have to worry about it...
However, I do feel the need to point out, 1e Players' Handbook, page 27,
"Back stabbing is the striking of a blow from behind, be it with club, dagger, or sword. The damage done per hit is twice normal for the weapon used per four experience levels of the thief, i.e. double damage at levels 1-4, triple at 5-8, quadruple at levels 9-12, and [I hadn't actually realized myself!] quintuple at levels 13-16. Note that striking by surprise from behind also increases the hit probability by 20% (+4 on the thief's "to hit" die roll) [which I also did not realize/remember!]."