Deutsch vs. Nederlands
I just started the Dutch tree today and completed the first exercise, but it seemed that even with my level 8 German it was very easy. Besides some different spelling, I was able to cruise through no problem. Does it continue like this? I am not sure that I want to invest the time into it if not, but if it stays this easy I may do it alongside my German learning. Any thoughts on that? I would love some helpful advice!
Dutch and German are both West Germanic languages, along with English, so they certainly have similarities. There are spelling differences, and German has cases, but they are similar languages indeed. As long as you are able to keep them separate while learning, you should be good to go.
As long as you practice in anything online, your streak grows. I originally started this streak in French, but after finishing my Spanish tree, I switched to Portuguese, then French, then Spanish, and now Dutch (I'll learn them eventually). This is all in 99 days, and my streak hasn't broken yet.
The one thing that pleasantly surprised me is that the genders almost always correspond in the two languages. So: het paard -> das Pferd, het kind -> das Kind, het meisje -> das Mädchen. German has always posed a bit a problem for me because of its three genders, but I find that learning some Dutch prior to taking the German lessons really helps because of the similarities.
I wouldn't do that if I were you. Finish your German tree first, and then move to Dutch. If you're keen on studying two languages at the same time, pick languages from different families, like German and French/Italian/Spanish/Portuguese, for example. If you want to do three, pick one Germanic, one Romance and one Slavic (or Asian, or Fino-Ugric or whatever).