I think that if "lacking both X and Y" and "lacking either X or Y" are synonymous, that's even worse! There are two clearly defined things I might have wanted to say (that each of X and Y was a thing I did not have, or that I did not simultaneously have both of X and Y), and I had at least hoped that the two "lacking" forms would take one of these meanings each :-)

I would probably understand and use these 2 forms more or less synonymously as well, in many situations. While I appreciate the distinction you wish to make, I suspect that actual usage is (as usual) far more varied and potentially ambiguous.

I thought "Lacking either X or Y" meant "There were two things X and Y, and I lacked one of them, but I'm not going to tell you which". Like "Add either substance A or substance B" doesn't mean add both, it means choose one and add it.

Well, yes, but that's an oversimplified example. The key point about "lacking" is that a listener may think of it as synonymous with "not having", at which point the both/either thing has a tendency to flip; "lacking either", by your definition, doesn't mean the same thing as "not having either", and if the listener has forgotten by the end of the sentence which verb form I used, it'll leave them uncertain of the meaning.

Nonetheless there seems to be a general consensus here that "lacking both" means "not having either", and that my post was therefore correct as written. It looks as if, had I wanted to say "not having both / lacking (at least) one", I'd have had to reword to avoid confusion...

Perhaps you should have said:


:) - guess who has been writing reports all day...

Aaaah. I think I was treating 'lacking' as a thing that you do in relation to the specific object, rather than a simple inverse of 'having'.

Language is a great thing.