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If there is one visible head and one hierarchy over two distinct Churches, and if both Churches have most of the same members, how can a Catholic separate from communion with the Conciliar Church without at the same time separating from communion with the Catholic Church?  

The answer is simple: because the True Church's doctrines, liturgy and traditions still exist, are still legal and there are bishops/priests to whom the faithful can go to follow them.  The Conciliar Church has setup a "parallel" system with "new understandings" of doctrine, with "new usages" of the liturgy and "updates for modern man" to Her traditions.  The True Faith is still obligatory, both morally and legally.  The conciliar system is not obligatory at all, neither morally nor legally.

Praeter If there is one visible head and one hierarchy over two distinct Churches, and if both Churches have most of the same members, how can a Catholic separate from communion with the Conciliar Church without at the same time separating from communion with the Catholic Church?  

Pax Vobis The answer is simple: because the True Church's doctrines, liturgy and traditions still exist, are still legal and there are bishops/priests to whom the faithful can go to follow them.  The Conciliar Church has setup a "parallel" system with "new understandings" of doctrine, with "new usages" of the liturgy and "updates for modern man" to Her traditions.  The True Faith is still obligatory, both morally and legally.  The conciliar system is not obligatory at all, neither morally nor legally.

But if the hierarchy and members of the two Church's are the same, a person cannot separate from the hierarchy and members of the Conciliar Church without separating from the hierarchy and members of the Catholic Church.  If the two are one, you cannot separate from one without separating from the other.

But let's apply what you wrote to the two Church theory to see how a partial separation could take place.

1) There is one pope and hierarchy over two Churches - the Catholic Church and the Conciliar Church.  Therefore, to remain a Catholic, it is necessary to remain in union with the Pope and hierarchy of the Catholic Church, even though the same men are also the Pope and hierarchy of the Conciliar Church.  If a Catholic separates from the Pope and hierarchy of the Conciliar Church (which is also the hierarchy of the Catholic Church), he is at least a schismatic (and probably also a heretic), and definitely cannot be saved ("submission to the Pope is necessary for salvation," etc.)

2) In order to remain faithful to the Catholic Church, one must retain the true doctrines, the traditional liturgy, and the traditions, all of which are still legal. To avoid the Conciliar Church, the Catholic must avoid any doctrinal errors and novelties contrary to tradition, none of which are obligatory or morally binding anyway.

3) Conclusion: To be a true Catholic and to save you soul, you must be in union with the Pope and your local Novus Ordo bishop, you must retain the true doctrine, traditions and liturgy, and you must avoid the errors and novelties.

Based on this exercise, who other than myself is a true Catholic?

1) There is one pope and hierarchy over two Churches - the Catholic Church and the Conciliar Church.  There
fore, to remain a Catholic, it is necessary to remain in union with the Pope and hierarchy of the Catholic Church, even though they same men are also the Pope and hierarchy of the Conciliar Church.  

Well, you'll have to define "being in union with".  I would define it as being in union with "Eternal Rome" first (i.e. true doctrine).  If the case arises where the pope/hierarchy are quasi-heretical, then 1) they are the ones not in union with Eternal Rome and unchanging, Divine Truth.  2) As Trads have always said, "You used to believe as we do.  We are the same; you are the ones that changed."  3) Faith is greater than obedience.  No man can make you sin, nor deny doctrine.  4) Truth is our Authority; the hierarchical authorities are not Truth.

If a Catholic separates from the Pope and hierarchy of the Conciliar Church (which is also the hierarchy of the Catholic Church), he is at least a schismatic (and probably also a heretic), and definitely cannot be saved ("submission to the Pope is necessary for salvation," etc.)

Traditionalists (of all kinds) have only separated themselves from the conciliar, parallel system.  This is not schismatic because 1) the conciliar, new-age "spiritualism" is not obligatory, and it's actually illegal and immoral (contrary to Quo Primum).  Since the V2 popes have NEVER commanded that ANY catholic attend the new mass, or accept V2 with any 'certainty of faith' so we are allowed, both morally and legally, to avoid these areas without schism.  Trads are under the law, even if new-rome acts like the law doesn't exist. 

2) In order to remain faithful to the Catholic Church, one must retain the true doctrines, the traditional liturgy, and the traditions, all of which are still legal. To avoid the Conciliar Church, the Catholic must avoid any doctrinal errors and novelties contrary to tradition, none of which are obligatory or morally binding anyway.

Agree.

3) To be a true Catholic and to save you soul, you must be in union with the Pope and your local Novus Ordo bishop, you must retain the true doctrine, traditions and liturgy, and you must avoid the errors and novelties.

You are in union with the pope and bishops by accepting the reality that they occupy the offices.  To denounce their heresies is not schism.  To avoid their scandals is not schism.  To attend private masses of real priests, who say the True Mass as legally allowed by Quo Primum is both morally and legally allowed and required.

Based on this exercise, who other than myself is a true Catholic?

The Truth is not based on numbers.  The "fewness of the saved" is both a reality and a sobering mystery.  Let's pray that we all persevere in the Truth we were mercifully given.