The OR clause is planned for AF 2018 SP2 and will be available in PI Builder, although it requires typing of the OR clause directly in the search box - for example: "Template:T1 OR Template:T2".
In AF 2017 and later, you can also include multiple templates IF you can identify them by name with wild card using the TemplateName filter. For example "TemplateName:T*". Note that this clause must be typed into the to search box - there is not an independent control on the search form.