forms Authontication for multimple website

hi
i create a web application that run in "localhost:1328" that use forms authentication for login users.
i create another web application that run in "localhost:1228" and i want to use "localhost:1328" forms authentication.
that means when user request "localhost:1228" if user not login go to "localhost:1328" for login and return to "localhost:1228".
if user request "localhost:1228" and before user login in "localhost:1328" then i can get userName in identity value.
i use the same machineKey for both web application and set domain="localhost:1328" in forms authentication in web config but i cant access user name.
so how can i use same forms authentication for both of web application?