<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by FlyerGoldII:
I was trying to book flights on the AC seat sale to W. Palm Beach this summer. Not only does AC not fly there, AC also does not codeshare on UA to that destination.</font>
Since UA always matches AC when they have a seat sale, you can simply book the whole thing under 'UA' flight numbers and you'll most likely get the same fare.