Handle concurrent HTTP requests with a generic .ashx
A generic ASP.NET ASHX handler can handle concurrent HTTP requests perfectly fine out of the box. This is partially thanks to the hosting infrastructure and ASP.NET itself. There's no need for all of this. I'm saying this because I'm not sure this is what you really want. 

But maybe you're referring to asynchronous handlers which means something else entirely, in which case I'd refer to this: http://msdn.microsoft.com/en-us/magazine/cc164128.aspx