Does any web site test framework facilitate fine-grained concurrent testing - unit-testing

Question

I have a legacy .NET application that is implemented using Application variables and makes heavy use of Session data as well. There are some anecdotal reports of bugs that seem to point toward concurrency errors, i.e. multiple sessions clobbering shared application-level data.
I want to develop some automated tests that let me control concurrent access in a fine-grained fashion, i.e.
Create two HTTP clients with fresh sessions
Request /my/page/1 with client 1
Request /my/page/2 with client 2
POST data with client 2
POST data with client 1
Issue parallel request for /my/page/results from both clients
etc.
Are there any libraries that make this sort of testing easier or will I have to roll my own to some extent?
I'm aware of Selenium and WatiN, but have not personally used either project. From reading the docs, neither appears to be a good match.
Perhaps the best option is just plain NUnit and making good use of the .NET WebClient class?

Answer 1

Clarification: What you want is a well-defined series of steps to be executed synchronously. This means your test code does not need to be multithreaded, more precisely, it must not be multithreaded, or you lose control over the order the steps are executed.
The only thing you need is two browser instances and the ability to dispatch the test steps to the two browsers.
To do so, I presume you can use either of Selenium or WatiN as follows (not verified, it's only in my mind)
Selenium (with WebDriver):
using (var firefox1 = new FirefoxDriver(profile))
using (var firefox2 = new FirefoxDriver(profile))
{
requestPageOne(firefox1);
requestPageTwo(firefox2);
postPageTwo(firefox2);
postPageOne(firefox1);
//// ...
}
WatiN
using (var firefox1 = new FireFox(url1))
using (var firefox2 = new FireFox(url2))
//// ...