a) Let us go then, you and I (b) Let us go then, you and me

Your example sounds right to me, but not the tip.

Whether to use me or I depends on what function it serves. In my examples, the first me serves as an object. "Let's keep this between you and me" means let's keep this to you and to me but not to anybody else.
The second example though, "I" serves as an subject. "Between you and I, we can get this done." means you and I can get this done and we don't need anybody else.

For the original question I feel that "you are me" is equivalent to "us" in its grammatical position. Since us is the object, we should use the object form "me" also.

Your example sounds right to me, but not the tip.

Whether to use me or I depends on what function it serves. In my examples, the first me serves as an object. "Let's keep this between you and me" means let's keep this to you and to me but not to anybody else.
The second example though, "I" serves as an subject. "Between you and I, we can get this done." means you and I can get this done and we don't need anybody else.

For the original question I feel that "you are me" is equivalent to "us" in its grammatical position. Since us is the object, we should use the object form "me" also.

The prescriptive rule is to use "you and I" in the same contexts
as "I" (i.e., as a subject), and "you and me" in the same contexts
as "me" (i.e., as an object). In "between you and me", since "you
and me" is the object of the preposition "between", "me" is the only
correct form. But English-speakers have a tendency to regard
compounds joined with "and" as units, so that some speakers use "you
and me" exclusively, and others use "you and I" exclusively,
although such practices "have no place in modern edited prose"
(WDEU). "Between you and I" was used by Shakespeare in The
Merchant of Venice. Since this antedates the teaching of English
grammar, it is probably *not* "hypercorrection". (This is mentioned
merely to caution against the hypercorrection theory, not to defend
the phrase.) Shakespeare also used "between you and me".

Betweeen You and I.
Between you and I? — Between you and I? — You should be ashamed of yourself.
First, the technical explanation: between is a preposition; it should govern the "objective case." (In English, that's a concern only with the pronouns.) A preposition can't govern a pronoun in the subjective (or nominative) case, even when there are multiple pronouns after the preposition.

That explanation should be enough for the serious grammar nerds. For the rest of you, think of it this way: when you have two pronouns after a preposition, try mentally placing each one directly after the preposition. "Between you" should sound right to your ear, but "between I" jars: "between me" sounds much more natural. Since it's "between you" and it's "between me," it should be "between you and me."

Ditto other prepositions, like for, to, from, with, by, and so on. If something is for her and for me, it's "for her and me," not "for she and I"; if Akhbar gave something to him and to them, he gave it "to him and them," not "to he and they." Try putting the preposition directly before all the following pronouns, and then use the form that sounds right in each case.

The problem probably arises from hypercorrection: it sometimes seems that you and I is "more correct" than you and me. It's not — at least, it's not always. Be careful. [Entry added 8 March 2005.]

between you and me
Between you and me is acceptable in standard English; between you and I isn't. This is because between is a preposition, and pronouns that come after prepositions are in the accusative case (here, me), not the nominative case (not I).

The same applies to a pair of pronouns that is the object of a verb: They've invited you and me to dinner is acceptable, They've invited you and I to dinner isn't.

The reason why expressions like between you and I have become so common is that people are aware that the accustive case is not correct for the subject of a verb ( You and I have been invited is acceptable; You and me have been invited is not), so they make the mistake of thinking it is not correct anywhere, and always use the nominative case.

If you are in any doubt, try leaving out the first pronoun of the pair. That will show you what case the second one should be: between I and they've invited I are clearly ungrammatical.

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HongHu, in the red part, is it right? Is 'I' a subject of the sentence or an object of the preposition 'between'? It's a little confused.

I think we should use 'me' as the object of preposition between.

I think anytime we need use 'me' because 'between' is preposition, it needs an object.