Can we view a single Asp.net page as one 'critical section' for identical http requests?

[itaitai2003]

How does ASP.NET process requests labeled with same sessionID/AuthID?
Is it in defined order, such as FIFO or undefined order?

For example: An .aspx where Page_Load()inverses a
bool session variable X.

First request starts at the top changes X and blocks, while a different
request carrying the same sessionID inverses X once again, blocks and
returns execution to the First request?.

Tnx

-Itai