How can I merge two sites?

Matt's answer:

CUTTS: Here’s a question from Rob in The Netherlands. Rob asks, “Can the ‘Change of address’ feature in Google Webmaster Tools be used to merge two sites, site ‘B’ becomes a part of site ‘A’, or is its use limited to moving a site from one domain to another? Do you have any advice for successfully merging sites?” Well, the “Change of address” form is intended to be moving from one site to another site. So I wouldn’t recommend that you try to be merging a bunch of sites or taking that sort of approach. If you do want to successfully merge sites, here’s what I recommend. Suppose you have two sites, “A” and “B”. And you want to take “A” and merge it with “B”. If you log in to Webmaster Consoles and prove that you’re on site “A”, you’ll be able to see essentially the full exhaustive list of all the links that we know about, so you can download as a tab or a comma separated value a list of all the links that come to site “A”. Now look through that and you don’t want to necessarily write everyone who has ever linked to site “A” unless there’s only, like, five or six people. But you can say, “Okay, who are the important people who linked to site ‘A’?” Maybe there is blogs, maybe there are new sites. Anybody that’s reputable that you think has high PageRank. And then I would say okay, write to those people and say “Hey, site ‘A’ and site ‘B’ are merging. The new page with be not here on site ‘A’ but here on site ‘B’. Would you mind updating your link?” And a lot of those high PageRank people, you know they want to have good links, they’ll be happy to update their link and point to the new part on site “B”. Then you want to do a 301 redirect from every page on site “A” to the appropriate part where that page is on site “B”. So don’t just do a 301 redirect from, you know, an individual page on site “A” to the root level of site “B”. Do it from whatever the URL is on site “A” to whatever the URL is on site “B”. That way, someone who’s landing on the old page will land on exactly what the new page is going to be. So in essence, you’re looking around at all the back links to site “A”, you’ve downloaded the full list according to Google and you can look at the ones according to Yahoo or Microsoft as well. And you’re trying to identify the important ones that you really care about and make sure that those definitely get re-written. And then in the meantime, you can go ahead and put 301 redirects in place to merge those sites. So that’s roughly how I would try to merge two different sites that you’ve purchased or two sites that you have, maybe you’ve purchased one and you’re moving them to a new site.

by Matt Cutts - Google's Head of Search Quality Team

Original video: