It’s amazing how difficult it is to Google how to do this exact thing, but it’s so simple. But there are already pages linked into your WordPress site that will display all your posts that are tagged with multiple things. You just need to know how to link to it! Here is an example:
Example: Link to all of my posts tagged both “css” and “website”:
That’s all there is to it! Just add a + between the tags you want to include. This should work with more tags, too! This page will be the normal tag page, but filtered only for posts that have BOTH tags. Alternately, if you use a comma (,) instead of a +, it would display all posts tagged CSS and/or WordPress. It’s a nifty trick that will hopefully help someone out!