Why.
Where is the tension? Elaborate please.
I thought it was pretty clear, but apparently it isn't.
Super-simple contrived example: having a public religious invocation as part of legislative proceedings (the issue at hand) seems to me to be a great example of where the tension is. Free Exercise clause, interpreted broadly, says that if the members of the legislature all want to have a prayer as part of opening ceremonies, there shouldn't be anything to prohibit them. Establishment Clause, interpreted broadly, would prohibit this as being a tacit endorsement of either a specific religion or religion in general. Ban it and you're arguably constraining the exercise of religion. Allow it and you're arguably establishing religion.
Frankly, I'm not good at discussing this sort of thing. Here's a better discussion of the issue:
http://supreme.justia.com/constitution/amendment-01/13-free-exercise-of-religion.htmlSome key lines:
The relationship between the Free Exercise and Establishment Clauses varies with the expansiveness of interpretation of the two clauses. In a general sense both clauses proscribe governmental involvement with and interference in religious matters, but there is possible tension between a requirement of governmental neutrality derived from the Establishment Clause and a Free-Exercise-derived requirement that government accommodate some religious practices.
Some more discussion of the issue (although a bit more muddied:
http://www.law.umkc.edu/faculty/projects/ftrials/conlaw/estabinto.htm). For even more discussion, Google it or ask your local constitutional law professor how many different approaches have been tried to reconciling the two clauses.
Really, this is why we have judges: to reconcile the problems that arise when laws conflict with each other.
