Thank you for your answer tomkmvp,
In fact, iframes were only an example. The matter is similar with parallel ajax requests (serialization) or server side caching methods (using xmlhttp object with appropriate cookies, it generates a deadlock).
But, in my test with iframes, I have tried to replace the .asp iframes with .txt files: They look served in parallel by IIS. In other words, my browser handles several parallel requests.
Also, in my test with server side use of the xmlhttp object, the IIS log clearly shows the requests serialization.
Moreover, I realize that the session object in ASP don't provide lock/unlock methods.
So... to bad, I have to rewrite a big ugly application :)
Thanks you anyway.