Since you have a head start on Norwegian, you should have little problem (except perhaps when tired, stressed, hungry, sick, etc -- as with anything else).
They are distinct enough to prevent that.
They are "cousin" languages, if languages were people. I speak French and Italian, which are "sister" languages -- far more closely related to each other than German & Norwegian. I started with French, added German after a while, and then added Italian. I feel and sound different speaking each language, even French and Italian. I do not mix them up (except as noted above - and then only briefly).
I do not speak Norwegian, but have read many articles on it. The grammar is quite different and core vocabulary is either different or distinct enough.
Note also: German is more inflected than Norwegian. Norwegian is a little bit easier to learn than German, for an English speaker. Norwegian requires 600 hours of study to reach competency (fluency takes years), making it a level I language (among the easiest natural languages for English speakers), per the American Foreign Service Institute. German, in level II, requires 750 hours.
one brief example:
I love you.
Norwegian (if I can trust Google Translate): Jeg elsker deg.
German (not from Google, but from me): Ich liebe dich.
German and Norwegian sound different, even if you don't know a word of them. You can learn both at this point if you choose.