You can mount a mashup by accessing the dev-hub part of your qliksense ... And yes it is possible to put several objects from different applications.
Follow like these:
First of all you need to fetch the data from all your apps (createCube method ‒ Qlik Sense). Eventually merge them together and create a simple filter (select Box or multiple selection box), Then the trigger need to call the select Value methods on all the source app selectValues method ‒ Qlik Sense .In this way a single selection impact all objects in your mashup, even id coming from different apps.
Yes you can merge them into one. It's mainly a problem with space, you might get a crowded page. To solve this you can use standard front-end methods with tabs, scrolling page etc. Try to avoid fetching Qlik Sense objects that are not visible, that is do not fetch the object on a tab until it is opened.
Do they use different apps?? You can use several Qlik Sense apps in a mashup, but selections will be separate, which can be confusing to users. If you want to coordinate selections you need to add some code, not very complicated, but has to be done. Another thing to consider is what happens for users with access to one of the apps used but not all.
Hope this helps