-- We could build a cabin like that.
-- Well, maybe not us, but two men could.
I would use we instead of us. Is us here grammatically correct?
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"Us" is more idiomatic. Consider this sentence:
He asked three people to dinner, but he didn't ask me.
"Me" is in the accusative here, because it's the object of the verb "to ask."
The sentence you're asking about is, in essence the same sentence. You are approaching it as if the full sentence represented by "not us" would be something like "Us couldn't build it." In fact, the whole sentence would more likely be unpacked something like this:
Well, not us, but two men could.
Two men could, but not us.
Two men could build it, but not us.
Two men could build it, but those two men are not us.
My theory is that "us" is accurate because this is an implied parallel construction, and the subject of both halves is "two men." So you would use the accusative just as you would in any other parallel construction:
I like her, but not him.
If the subject or verb was included, it would eliminate any ambiguity, and in that case, with the appropriate addition, either "us" or "we" would be idiomatic:
Two men could, but we couldn't.
Two men could, but they're not us.