The way you have been typing it needs to match everything you type which I am sure is not what you want.
it should be something more like:
Code:
"title1" or "title2"
title:"title1" or title:"title2"
The keywords and/or are treated as logical operators (if you want them to match a string they need to be inside quotes). The first variant looks anywhere in the searchable fields. The second one limits it to the title field. The default is that matches are the "contains" type although it is possible to do exact matches. More details on the search syntax are in the Calibre User Guide.