I think #4 makes a brilliant point when she points out the example of the Roman Empire and the way that the Jewish system of law existed within the Roman system of law. This is an excellent example as it shows us the many difficulties that emerged from such a system, and how it created lots of tension between Romans and Jews. The right to sovereignty is one that certainly makes governance much easier than when there are two different systems each vying for control and dominance, but it does not necessarily mean that such a system is unworkable.
I would agree with above posters that we actually do operate with more than one system of law. We do have Federal and state law, but we also have moral law. Those who are religious would call it the law of God, as stated in the Bible (most specifically, the Ten Commandments). However, those who are not religious STILL operate morally. They do have a very strong sense of justice, and most would agree with the basic Commandments--don't kill, don't lie, don't envy others; don't commit adultery; honor your parents; don't steal; etc. Most people agree that these are things no one should do, although these laws are broken from time to time...
As has been pointed out above, there is a duel system of laws in this country; both State and Federal. As far as secular law vs. religious law, I think they can co-exist equally. The First Amendment to the Constitution prohibits an establish ment of religion but guarantees the free exercise of religion. There have been instances in which the two have conflicted; but there have been instances in which other constitutional rights have conflicted. As long as there is no major infringement on the civil rights and liberties of others, there is no reason why one cannot consider himself to be bound by a religious law.
The Constitution and the Founders were explicitly clear about the separation of Church and State, and over time this has come to mean two things with regards to the existence of religious law. First, it means that the laws of the federal government are more powerful than any religious law in the eyes of the courts. Second, it means that religious law cannot be influenced or controlled by the government. This may seem contradictory, but actually it has evolved into a system of mutual respect that works quite well.
The protection the churches and religious laws still have within the system is in the First Amendment. Religious laws that call for observances now considered illegal, say the use of Peyote by the Apache tribe, are protected by the courts even when it conflicts with federal and state laws. So the system, as designed, allows both sets of laws to exist in concert with one another, and even allow religious law to trump the state law on a case by case basis, once in a while.
Historically, there are and have been some societies that have or have had two disparate sets of laws, usually secular and religious. An example of one such is the coexistence in the Roman Empire of Roman law and Judaic law. The New Testament gives a good illustration of the function of these two together when Jesus is tried by the religious legal body called the Sanhedrin then taken before Pontius Pilate to be tried by the Roman authorities. Pontius Pilate had the authority to override the ruling of the Sanhedrin but did not dare, hence his famous public handwashing to absolve himself of any guilt in an execution that he fully disagreed with.
This record illustrates the conflicts and difficulties of having two disparate sets of laws--laws that can try and convict on two wholly disparate sets of principles and values. It may be argued that the division of authority between federal, state, and local bodies is not a division between disparate ruling bodies but rather semi-autonomous subdivisions of one ruling body as all are bound by the same principles and values--all are bound by the laws of the Constitution and the principles and values of the earlier document, The Declaration of Independence.
This question would be easier to answer if it were more specific. Are you asking about two different systems of secular law or are you asking whether religious law and secular law can coexist. Given your previous question, I assume that you are asking the latter question.
If this is the case, I would argue that religious and secular law can coexist so long as it is clear what matters may be placed under the religious law and what matters are controlled by secular law. For example, it seems that it would be acceptable to allow churches to set and enforce their own laws about who is allowed to be a minister in that church and how such ministers must behave. However, it would not be acceptable for a church to set up laws (about child abuse, for example) that conflict with ones that are truly the province of secular law.
So, I would argue that these two types of law can coexist so long as secular law is clearly supreme and there are clear boundaries between the two.
The questioner does not indicate the governing authority of the nation... a democracy, a church state, a totalitarian regime...
The concensus seems to be that, in a democracy, even one founded on Judeo-Christian beliefs, two systems of law can co-exist... secular and religious law, with the caveat that secular law "reigns supreme" over religious law. This makes for a rather tenuous co-existence at times. One only has to look back at history to see that this co-existence is not always peaceful, and more than once the heavy boot of secular law has trampled over religious freedoms.
# 5 makes a couple of statements that I do not agree with... "the laws of the federal government are more powerful than any religious law in the eyes of the courts." This is the heavy boot of secular law trampling religious freedom.
"...religious law cannot be influenced or controlled by the government." This is a contradiction and an untruth because in fact, secular law can and does influence relgious law.
So perhaps... even in an enlightened democracy... two systems of la cannot exist?
I do not believe federal and state laws are two different systems... they are merely "degrees" of the same system, with "local control" sometimes trumping federal, but their principles of operation are the same.
In a church state or a totalitarian regime, two systems of law could not exist.