My personal approach is that it depends on the logic, each has it's pros and cons.
Let's say an activity has a list fragment and a details fragment, if theses fragments you are pretty sure they will only be used with this activity then you can have one presenter, but what if you want to use the detail or the list fragments in other activities, or if it's a list of people and the detail for each person, then you can open the logged in user detail view ( from a place other than the people list) ? you will use the same presenter of the activity ( not that good architectural wise ) so in this case having 2 presenter in my own opinion is a better approach!
To cut it short, there's no rule for it, it just depends whether if you are going to use the views in other scenarios or the view is only attached to this Activity.