On iOS 6.0.2. Can't see nor search my Sent emails other than the ones I actually sent from the iPad. I'm using Yahoo Business Mail on IMAP settings.
The other device that is sending email probably isn't saving them to the Sent folder.
If you're using the Yahoo! website to send mail, you can go to Mail Options > General > Sending Messages to fix this. See the their support article for screenshots and more info: http://help.yahoo.com/l/us/yahoo/smallbusiness/bizmail/options/options-17.html
If the other device is using OS X Mail, go to Mail > Preferences > Accounts > Mailbox Behaviors and make sure "Store Sent messages on server" is checked.
For Mail on iOS, Settings > General > Mail, Contacts, Calendars > (your account) > Account > Advanced and make sure your sent mailbox is on the server.