Fortunately, your Mystery accounts share the same login credentials! Whether you're signing in directly at www.mysterywriting.com or www.mysteryscience.com, you can easily toggle between the two sites.
To switch back and forth between sites, click on the bolded links at the top of the homepage. This layout will look the same on Mystery Science and Mystery Writing.
If you've already scrolled to the bottom of the page, you can click on the links at the footer.