Allow multiple taxonomy term values on term listing page
I've found the path taxonomy/term/203+221 (which will yield the nodes tagged with either 203 OR 221) is possible by making a slight change to the Contextual Filter of the taxonomy/term/%tid view (which you should have enabled in Structure > Views > Taxonomy Term. 

Expand the Advanced section of the view configuration, click on the first Contextual filter "Content: Has taxonomy term ID (with depth)". 



You will see a checkbox that allows multiple values, which is checked by default.



However, to get this working I had to scroll further down to the "Specify validation criteria" section and change the "Filter value type" to "Terms separated by a , or +".  



Now you should be able to use multiple term IDs in your URL, combined with a "+", to see all nodes tagged with any of the IDs in the list. You can test out various URL arguments using the Views Auto preview: