Tá an bheirt chailíní ag rith - "Both girls are running"
I would also have to quibble with scilling about the acceptability of "a couple of girls". In Ireland, that would be understood as "a few girls" - it is not restricted to "two girls". "A couple of days" isn't "two days" and "a couple of people" can be more than two.
On the website, when you open a particular skill, if you scroll down below the five or six "sections" in that skill, you'll find "Tips & Notes" for that skill, if there are Tips & Notes for that particular skill. They may not be available in the various Duolingo apps, or in the browser on a phone.
Edit - Since the website redesign the Tips & Notes are no longer displayed when you open a skill - now you have to click on the lightbulb icon over the Start button when you select a skill.