New site based on data from another - the best system architecture?

So here's the thing:

1. I have a website, let's call it Catalogue. It is an advanced product catalogue with a lot of items in it. It works on php based CMS, it wasn't done by me and is fairly complete and complicated system. What is important, it has user authentification system and a lot of users in the database. You can not access most of the products unless you are logged in.

2. I want to have another website and I want to build this one on my own. It will contain some additional data for the products, extended download section and some media too. I want to build it with Django.

This other website is going to use Catalogue's database a lot: - it needs to authorize users using their login/pass from the Catalogue - It will need customers data to customize their views.

I don't have big experience in building more advanced systems. Could you, experienced programmers tell how would you do that?

Specifically, I'm concerned about database: should I only use the original database? And merge all django related stuff in it? Will it affect Catalogue? Is it generally good idea?

As I've mentioned I'm going to use user data from Catalogue's database to authenticate users. Are there some security concerns with this idea?

I intuitively feel that I should use one database rather then two. Please correct me if I'm wrong.

Please let me know what you think about such idea. Maybe it's not worth pursuing and should be done in a completely different way?

Thanks,

C