Integrating multiple installations?

Hi! I’m not very familiar with wordpress capabilities and would like someone to tell me if this is possible;

I’m going to use wordpress to update posts on my site and also use a different wordpress installation for a gallery solution –it requires a completely different theme and stuff.

Can the different installations require the same user names for logging in and subscribing and such? How could I integrate the instillations so you wouldn’t have to register twice for the site?

One installation is going to be in the root folder and the other is a sub domain!

Thank you for your time!

(If this has been asked before, I’m really sorry! But I couldn’t find a WP2.1 thread about it)