membership on more than one multisite setup

This is a little confusing, but I will do my best to explain what is rattling around in my brain.

I'm working on a series of articles about different ways to use WP with Multisite, and this is one of the ones I want to work on, but whether it is possible or not, I have no idea.

There will be one main domain, for example,, and a load of sub-domains, for example,, &

The main domain and all the sub-domains will have a separate multisite installation.

Would it be possible to have people sign up to one of the sub-domains and automatically create that member, with the same details, on the main domain.

If possible, it would also be good if we could pull in information, such as posts, from the sub-domain and place it on the main domain.