Hi there - yes there is but not in the settings of the gadget. You do that by adjusting the size of the placeholder so that only one tweet fits in there at a time. OR, you could adjust the size of the font of the tweets to that the font is so large that only one tweet fits in the placeholder at a time.
That's a good point. Yeah, I'm sorry, but there isn't a way in the configuration to show just one at a time, so the two options I provided are the best. Well, the best I can think of anyway - maybe someone else here in the community will have a good suggestion for this.