Dutch And German
I am thinking of studying Dutch and German at the same time, would this be a good idea as the two languages are alike? So it would get confusing on which word belong to which language.
Pro: The grammar and a lot of vocabulary are similar, so you'll have an easier time learning things when you can derive one thing from another thing in the other language.
Con: You'll mix up the words
So, kind of stating the obvious here. Anyway, I think it's worth a shot if you're interested in both languages individually. If you're only interested in one language and looking to get the other one as a freebie, I'd say don't bother, it would still be a decent amount of extra work.
I would suggest you try and get your German to a decent level (and by that I mean beyond what duolingo can teach you) and start Dutch once you're at least somewhat conversational in German. There's still a high chance you'll mix German into your Dutch, but the opposite isn't as likely to happen.
At level 18 German, I think you are likely ready to learn one of German's sister languages. If anything, German will help you loads and when it's not like German, it's often like English or both. Also, a minor amount of confusion won't hurt you in the long run. Just ignore it and keep going. Learning related languages help each other way way more than they hurt each other.
Being Dutch with good knowledge of German language, I would say it's a bad idea. Both languages have a lot of similarities, but at the same time are different. I guess Dutch may be a bit easier to learn, German is probably more useful. Personally I would recommend to learn Dutch first and then German, but the other way round may work as well. Trying both of them, is probably very confusing.