2 wordpress blogs with one registration

Hi,

I have installed a wordpress blog on my main domain. Now I want to add a classifieds to my website. For this, I am thinking of using another blog in my sub domain with classifieds theme.

I have been reading articles about having a blog in sub domains may not be seo friendly and may not produce effective results. please advice me on this.

Next, As there are 2 blogs, I would want my users to register(sign-up) and log in in one blog and still be able to use the other without separate registration.How to do this? I use the the latest version of wordpress.

Thanks
Ana