Here is how to set up the Gmail "send as" feature on your iPhone. This will enable you to sync one Gmail account while retaining the ability to send from all forwarding addresses linked to that account from your iPhone.
First thing you have to do is make sure IMAP is enabled in your Gmail settings and that your "Send As" addresses are already set up in your Gmail account.
Then you must set up your account as an IMAP account on your phone.
To do this, go to Mail, Contacts, Calendars in your Settings, click Add Account, and choose other, NOT Gmail.
Once you have done this, click "Add Mail Account."
Enter your Name, The Primary Gmail Address you are using, Password and Description.
On the next page, make sure your incoming server is imap.gmail.com and your outgoing server is smtp.gmail.com
In the username field(s) on that page, make sure "@gmail.com" is included in the useranme.
Enter your password, and add the account.
Once you have done that, you need to go back into your account settings.
Go to settings > mail, contacts, calendars > "your gmail account" > "Account" > "Email"
That will take you to this page, where you click "Add Another E-Mail"
Enter the e-mail address(es) you want to "Send as" on this page and you're done! NOTE: it will take anywhere from several hours to a day to begin working. I set mine up at night, and the next morning I was able to send mail from my phone again.
One it is set up and working, your "send as" addresses should display as regular choices in the From menu in Mail:
This should work for the iPad as well.
Cheers!