How can I find out which universities allow a sandwich (stapler) PhD thesis?
The department where you are going to submit the thesis probably has a document of PhD/thesis regulations that specify if such a thesis is allowed. For example, my department's regulations have the following statement:


  [...] Scientific publications may form a part of a doctoral thesis. If the doctoral thesis consists of several scientific papers, a presentation of the guidelines of the papers submitted has to be added in an appropriate extent. [...]


i.e. a sandwich thesis is possible here. So you should find out if your favored departments have similar regulations and have a look in there.