WordPress with two themes or two installations?

I’m just starting using WordPress (I’m a newbie) and I need help deciding the best approach for setting up my site.

I have two different areas within the same website – articles & photos (or pins like pinterest). The pins portion will use the wordpress theme iPinPro. The articles portion would be a different theme.

This is what I want to be able to do:
1) Single Login – I need to password protect both areas – preferably with one login for both – each user needs to have their own login/password. (Currently I use .htpasswd and I’m not sure if that plays with WP or not)

My users each need logins to be able to access the WP content. In the iPinPro theme they need to be “Authors”. But on the article side, they are just “Subscribers”.

2) Separation of data Articles need to show only on article side – not on pin side. Pins would be best to show only on the pin side (but I could handle if they showed on the article side – I think).

3) Single search – It would be nice (but not mandatory) to have a single search that allows users to search both areas at the same time.

How do I do this with WordPress? One installation w/two themes? Two installations? One database or two? If one database, two separate tables? and does that mean it would be two searches?

Any direction/ideas would be appreciated.
Thanks!