How can I sync sent mail from another device on my iPad?
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.