@ jonnyapple and studio, I've just finished running my own unofficial test for frames per second using the EN-EL15 battery along with 6 new AA Duracell batteries in the battery pack (I don't have a second EN-EL15 yet).
Role played by card in Slot 2: Overflow
Image Quality: JPEG Normal
Image Size: Small
AF - C
Shutter priority: speed 1/1600 sec
I used my AF-S Nikkor 35mm f/1.8G lens (@ f/1.8) in a well lit room aimed so that a brightly lit lamp was in the field of view.
To sum it up I'll say the results were as advertised. With the EN-EL15 alone (battery pack empty) I got 6 frames per second. With the fresh Duracells in the battery pack I actually got between 5 FPS and 6 FPS. In other words the shutter speed was more sporadic with slowing and speeding up. I don't have a battery tester so I don't really know if the batteries were at 100% even though they were new.
Aside from all of that, I don't think the system was designed to pull power from both battery sets at the same time anyway since the menu calls for you to decide which battery set you want to use first before moving on to the 2nd battery set. After seeing the above results I've decided I will eventually get a 2nd En-EL15 for the battery pack.
I hope all of this helps.