The two chicks are really growing, and one in particular does now seem larger than the other. Chances are that we have one male and one female, the females being larger than the males.
In this video, the larger one is nearer the camera.
Between feeds, they usually lounge about or sleep, and now that they are larger, the parents are no longer able to brood them both, so often perch on the side of the tray keeping an eye on them.
Ringing is due to take place in the next few days, during which measurements will be taken to try to work out whether they are male or female.