You can get up to 1600 updates for each user (3200 limit!)
and compare XMLs
but watch your Rate limits 
http://api.twitter.com/1/account/rate_limit_status.xml

Other approach is to use something like TweetGreed with search for
both users
but there's a 100 updates limit - basically is you have 100 between
you - invite and if less - don't

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