A dual capture mode with a combination of 5MP and 16MP images would be a significant step down from the Lumia 1020's 5MP and 34MP dual shot mode, but would still allow for some use of both zooming (x2) and reframing (1/4 of image). Both of these would be notable feature points in a phone with a relatively small camera bump and would help the Lumia 1520 stand out from other large screen devices.
The 16MP high resolution image size suggest the Lumia 1520 will have a camera with a native 20MP resolution. However, because Nokia provides true 16:9 and 4:3 aspect ratio modes in its cameras, the corner portion of the sensor is not used, hence the lower real megapixel output (16 MP in the case of 16:9 mode, and likely around 18 MP in 4:3 mode).
As with all leaked images this information should be treated as unconfirmed until the formal launch announcement is made by Nokia. With that said, it does now seem clear that the Lumia 1520 will have a 20MP camera and will use some of the same techniques as the Lumia 1020. However, the sensor characteristics (size) will almost certainly be inferior to to the Lumia 1020. Notably, a different sensor is being used compared to the Lumia 920 / 925, so how the Lumia 1520 will compare with those devices will be an interesting question to address once the device is formally announced.