Dideved - 1:25 pm on Apr 9, 2013 (gmt 0)
The first part of your request -- to redirect from olddomain to newdomain -- is actually straightforward, and the Apache documentation covers exactly this scenario.
But the second part of your request -- to also have a different site running on olddomain -- presents a problem. What happens if your new site needs a URL path that your old site used to use? You can either redirect to the original content or you can show the new content, but you can't do both. In particular, olddomain's homepage won't be able to redirect to newdomain's homepage, because now olddomain needs to serve a new and different page.
It's also entirely possible that Google will transfer your PR to newdomain rather than retain it at olddomain. Here's what Google has to say about moving a site.