qsappend|QSA' (query string append)That looks like setting up for trouble in this case.
This flag forces the rewrite engine to append a query string part of the substitution string to the existing string, instead of replacing it.
(.) matches a single character (the question mark) and should result in http://foobar.com/ redirect?1=five?1=one&2=two
Try
RewriteRule (.*)1=one(.*) http://foobar.com/$11=five$2 [P,NC,L]
Other than that, I'm not sure if the condition rule triggers: it should be "RewriteCond", and one of the parameters is missing.