Your iPad will set your first added email account as your default email. This means that it will use this account to send from whenever you share content like photos or links from your iPad via email. You can easily change your default email to another account if you so wish.
To change your default email account, head to Settings -> Mail, Contacts, Calendars and select Default Account from the right side panel. From there you can check off your default account by tapping on it.