how to share authentication info??

[Tom]

Hello friends
I have 2 webs sites on same domain say

http://app1.mydomain.com

http://app2.mydomain.com

App1 use custom validation, via a custom login page user
can enter their WinNT username and password and enter into
the application. It validates the username and password
against LDAP domain.

App2 use the same LDAP domain/NT domain for basic
Authentication, user can enter same Win-NT account name
and password to continue.

Now what I want now, is user need not need to login twice.
How I can share user authentication information between
these 2 sites.

The both sites are used internally on an intranet server
inside an organization.

Regards
Tom