If I have a domain name say stackexchange.com and I want to host 2 sites X and Y as subdomains of that site.

Will x.stackexchange.com help out y.stackexchange.com in page rank? Or will google treat them as 2 separate sites as far as page rank is concerned?

Is there any SEO benefit at all?

The question stems from this post and its answers and comments by the way: http://meta.webapps.stackexchange.com/questions/624/webapps-stackexchange-com-verus-nothingtoinstall-com

Comments like this one and others:

No doubt the site will be great either way. But all things being equal questions from a "smaller site" (with less traffic and less overall linkbacks as far as Google is concerned) will have a lower Google ranking than if the site is "seen" as part of a huge network (made up of all the SE sites combined) with ONE top-level domain. Is it worth it? – Robert Cartaino♦ 2 days ago

Or this one:

Is it worth sacrificing the network effect of being on the Stack Exchange Network AND the lower Google ranking on every question (smaller sites will have lower rankings than a large network) to have the independent domain name? That's what I am asking. – Robert Cartaino♦ 2 days ago

Or this one:

@alexanderpas correct, and nothing will destroy that search ranking faster than splitting pagerank across 25+ unique domain names. – Jeff Atwood♦ yesterday