A General answer:
There is no way to "concatenate" namespaces, but you can give them different prefixes
<html
xmlns:fb='http://www.facebook.com/2008/fbml'
xmlns:og='http://ogp.me/ns/fb#'>
Then use fb:tag for facebook tags and og:tag for ogp.me tags
A specific answer:
The recommended use of Open Graph protocol is not via the xmlns attribute but via a prefix attribute ( http://ogp.me/ ) So the code that uses both, will look like this:
<html xmlns="http://www.w3.org/1999/xhtml" dir="ltr" lang="en-US"
xmlns:fb="https://www.facebook.com/2008/fbml">
<head prefix="og: http://ogp.me/ns# [YOUR_APP_NAMESPACE]:
http://ogp.me/ns/apps/[YOUR_APP_NAMESPACE]#">
<meta property="fb:app_id" content="[YOUR_APP_ID]" />
<meta property="og:type" content="[YOUR_APP_NAMESPACE]:recipe" />
...
Example courtesy of https://developers.facebook.com/docs/opengraph/tutorial/