here 2 users will get 2 different instance of the action
class. each one will behave in their way as said in
configuration file.

In this case both the users will get saparate instanceses
of the same action class.and both will work independintely.

For every request........crossponding instance is 
created.....
Means n no of rquest=n no of instance.......
And for every instance one thread is allocated for 
that......and then......somtimes may chances of deadlock...

I do not see a single correct answer.

In Struts1, Action resources must be thread-safe or
synchronized. So Actions are singletons and thread-safe,
there should only be one instance of a class to handle all
requests for that Action. The singleton strategy places
restrictions on what can be done with Struts1 Actions and
requires extra care to develop. 

However in case of Struts 2, Action objects are instantiated
for each request, so there are no thread-safety issues. (In
practice, servlet containers generate many throw-away
objects per request, and one more object does not impose a
performance penalty or impact garbage collection.)

reference:
http://www.javabeat.net/qna/690-struts-1x-vs-struts-20/