Yes, they can.  But what happens when they do, can be to the detriment of secular societies.

The Vatican and Muslim leaders can get together but they do so to: “act together in defending and promoting the moral values which are part of our common heritage.”  and Proposition 8 passing is such an example of such promotion of moral values.

As well as pushing for censorship over free speech in the Danish Cartoon controversy:

“The Al-Azhar-Vatican committee issued a final communique… Members of the committee called on the media not to misuse the freedom of expression to insult religious beliefs and symbols, MENA said.”

It’s always nice to see people get together, except when their idea of the world isn’t good for you.