W2K PPTP VPN isseus

We have two site with same domain name. The problem is when site a share
network drive, site a user PC map site a share drive, but when user using
PPTP VPN connect to site b , the user cannot access the site share drive.
When user disconnect the PPTP VPN, the user is possable access the site a
share drive. I know the direct solution is rename the domain name in site a
or site b, but it is very difference. I want to know any solution to solve
out the problems.

Thanks
J