Both are compound verbs actually, 'mitreden' and 'mitgestalten', the prefix mit- works the same as co-, as far as I know. Although I'm not sure if duolingo really meant 'mitreden'. It's possible they mean the talk and therefore co-create, co-design whatever they're talking about. If that's the case, 'mitreden' might be better.
So "mit-" can also be the prefix of a separable verb? I had already encountered a bunch of separable verbs in Duolingo: aufhören, abfahren, ausgehen, just to name a few. But this is the first time I met something starting with "mit-."
What you said just helped clear things by a lot. Thank you! :D