You should enter both. Since you already have Janisch-32 listed as his wife, I would complete that by entering the children from that marriage.
While on Joseph's profile page, go to edit...When the edit page is open, you'll see Spouses: on the right side of the page. To the right of that, you'll see (add/remove a spouse). When you click on that, a page will open to add a spouse. Enter Janisch-37 in the box requesting WikiTree ID, then hit GO. Janisch-37 will then appear as second spouse when you click on Add Spouse for Joseph Marek at bottom of page. When you're ready, you can add the children for that marriage.