2.0 Auth library recommendations / session takeover

[eluser]wwwald[/eluser]
It's been some time, but I'm starting on a new web project and planning to use CI Reactor (have been working with 1.7 before). I have 2 questions regarding authentication...

General question: what is the recommended / most stable auth library out there, that works with Reactor? I've been using DX_Auth before but it seems to be problematic to get running on CI Reactor, judging from forum posts.

More detailed question:
The website I'm working will have to interact with another website, but we connect to a shared MySQL database. One of the interactions concerns a unified login system: when people login on the other website, my site should take over the session without requiring a login again.
How would I go about this? I suppose this involves writing a custom session loading mechanism, right? Does anything like that exist already?

Thanks a lot for your help,
wwwald