I wouldn't. Compare Dutch and German with Portugese and Spanish, Swedish and Norwegian, Russian and Belarussian, Italian and French, English and Welsh. They might seem similar at some aspects, but you don't want to learn them at the same time. It creates confusion and you might pronounce Dutch words in the German way, which is really weird to Dutch people (and immediately heard)
I'd learn them seperately, preferably even one by one.
If you mean writing one notebook entry in two languages, posting it in the Dutch section, then no, that's not a good idea :)
Dutch and German are different languages of course.