First of, I recommend you install only those apps that really enhance your learning. Listening, reading, singing, dancing... great! Leave aside those apps that deal with grammar for now. You will have a much easier time with the grammar once you're already fluent in German. (This has to do with the development and structure of the brain.) Personally, I would also leave aside any apps that feature repetitive learning for vocabs. Lastly, if you're motivated to learn German for two hours per day, do it in 10-15min segments across the day. (This has to do with subconcious sorting processes of the brain.)

As I said, I don't recommend using any app at all for vocabs because our brains cannot sort in isolated pieces of information (e.g. vocabs). If you are however committed to learning vocabs by heart, choose the app that lets you combine pictures with the words / expressions. Pictures are the mind's default way of thinking, and it has a much easier time processing pictures than single words that appear out of context.

Thank you, Jan
I should leave the apps that deal with grammar, yes, but for the rest, you didn't really answer my question -even though you answer is very interesting and appreciated: Should I use one app per day (let's say I have two great apps for vocabulary now) or use them both -in intervals, as you said.