Do you want to do multiple tag/category queries in WordPress by URL?
Use , (Comma) for OR operation
Use + (Plus) for AND operation

Suppose to get all posts tagged css or jquery then:

https://techbrij.com/tag/css,jquery

To get all posts tagged css and jquery

https://techbrij.com/tag/css+jquery

For those of you not using “pretty” permalinks, then it would be:

http://mywebsite.com/?tag=tag1,tag2
http://mywebsite.com/?tag=tag1+tag2