merging 2 site under WP3.0 RC1

I did so and it works (not for registred users)
Update the 2 wp to 3.0
Choose the database who have to supoort the multi site. Install the multi site and create the second blog with the super-admin, so you get the prefix wp_ for the first blog (en) and wp_2 for the second (fr).
Rename all tables from the second blog (fr) in wp_2 on the old database, backup the base and import all on the first database.
(some plugins running under 2.9.2 can rename the tables for you. In this case you have to rename the tables before update the database to wp 3,0)
Thierry

Hardcore geek way:
http://bavatuesdays.com/importing-a-single-wp-blog-to-a-wpmu-installation/

Longer but less geeky way:
http://wordpress.org/support/topic/404321?replies=5#post-1532281