The 360 version of Mass Effect is only sitting at 2.08 million. It's highly unlikely half of those people will buy the sequel the very first week.
It probably has less of a chance at breaking a million week 1 than Uncharted 2.
Edit: Same goes for Splinter Cell. Double Agent only managed 1.14 million on 360. The only Splinter Cell that managed substantial sales was the first, totalling 2.91 million, but even for a game that managed those figures a million first week is highly unlikely. Just look at God of War. God of War sold 3.21 million, but the sequel only managed a ~600k opening week.







