I want to express an assertion that A cheated or C cheated. Can I say:
- Maybe one of A or C cheated.
- Maybe one of A and C cheated.
Or is neither correct?
I would drop the "one of" and go for:
If it is really important to convey the idea that either of the two may have cheated, but certainly not both, you can say:
Either in this sentence makes it clear that you do not mean both.
As hinted in the remark (thank you!), in case you have more than two options, the use of either is incorrect. In that case, you can use
Again, that does imply that you do not think it likely that A and B have cheated, but not C.
You could also use
But I think you might risk creating confusing with people that parse that phrase in a very strict way. Was it (A) that cheated, or (B and C)?
Personally, I associate
Since a choice is implied here, I would prefer
If you want to emphasize that you have a group of people (A, B and C), out of which one may have cheated, use
You might want to use the Oxford comma in order to avoid confusion about (B and C):
"Either in this sentence makes it clear that you do not mean both." I don't agree. See http://dictionary.reference.com/browse/either?s=t.
In logic there is the exclusive or - either a or b but not a and b - and this is what you were trying to make clear with your "one of" construction, but it is difficult to be precise in English, unless the context makes it clear that the (two) options are mutually exclusive as in right and left, up and down.
I am not clear how certain it was that anyone cheated in your scenario, but I would go for "one person cheated - either A or B" or "one person, either A or B, cheated". You could omit "person" but it would by stylistically odd.
You could also go for "either A or B cheated, but not both".
Although I agree that "oerkelens"'s way of saying it is better, I think "and" could serve and "or" should not, because I'd say "Maybe one of them cheated", and "them" is definitely plural.
If you are sure there was cheating do not say "maybe" just say "one of A or B cheated" or say "either A or B cheated". Using the word maybe says to me that there might not of been any cheating.
If you are sure that there was cheating but not sure how many did, you could say "one or both of A or B cheated". Note that you could of said "one or both of A and B cheated" where the 'and' specifies a logic set of A and B that one or both from the set, could have cheated. If you use 'and' I expect that most people would subconsciously jump to the assumption that both did cheat, even though that is not what the logic of the sentence says.
If you are unsure that there was cheating, but if there was, there was only one cheat, you could say "one of A or B might have cheated"
If you are unsure that there was any cheating, or one cheat, or both cheated then you could say "one or both of A or B might have cheated".
"Maybe A or B cheated, but not both." gets the logic across and sounds natural. If it is possible that neither of them cheated, say "maybe A or B cheated, or neither, but not both." If it not possible that neither cheated, drop the maybe. "A or B cheated, but not both."