VBseo with DDDS optimise for Search Engine

I am planning to create another site as a parent site SiteA.com of my current site e.g. SiteB.com.

I will be using Vbulletin.org's Different Domain Different Style to do it

I am pretty sure that I will be able to get into www.siteA.com/category1 and www.siteB.com/category1

How can I limit the fact that those category in siteA can't be looked up on SiteB by search engine spiders?

Do I need to do something to robots.txt?

This is a horrible idea in terms of SEO.

Here's what happens.

Someone searches for 'widgets'.
you have a widgets thread.

but, instead of being a 100% result, you now share your 2 pages at 50% each... which means someone who hits an 80% share will out rank you.

For optimial SEO,
pick 1 domain.
park the other one on it
and 301 it to the real domain.