Hello aksher@cisco,
Here's a conceptual explanation.
If topology looks like so
10.18.12.x----Pix--------Internet---------Router/Pix---10.18.12.x
What you'll need to do is nat 10.18.12.x to some other space on both sides. For example, you could nat 10.18.12.x to 10.18.200.x on the Pix side and 10.18.201.x on the other side. Then when you configure IPSec you'll have different networks and traffic should be able to sent without issue.
Hope this helps! If so, please rate.
Thanks