Swipe down on screen to show notifications. Tap the Tap to Tweet button.
Tap the twitter username to display all your twitter accounts.
Select the primary twitter account from the list of twitter accounts.
Add a message to the newly selected twitter account and press send.
Now you can use Siri to post to this account by saying 'tweet' and then your message. For example, you might say 'Tweet Good morning friends'. And you will post 'Good morning friends' on your account.