A frequent question people ask about SimplyTweet is - what is that button in the centre with all the checkmarks?
That's referring to what appears when you swipe across a timeline:
It's the reply all button. It lets you conveniently reply to several people in a single tweet.
Here's what you do: press the reply all button, the swipe menu will disappear, revealing the timeline again. Notice that the top right button has changed and there's now a column of checkmarks on the right:
Touch the tweets which you want to reply to and their respective checkmarks will turn green (indicating they are selected):
Press the top right hand button to reply to all the authors of the selected tweets: