Mine were 2 1/2 weeks apart, Dutch Hookbill ducklings. But, I had only one of each age. I got the second one after a very bad hatch, to be a friend for the first one.
The best mirror I found in the auto section of Walmart. It is suppose to be a replacement for your side view mirror. I...
Mine only needed to stay separated for a few days. Just make sure the little one has a place to escape when you remove the barrier. I used the brooder plate. It was too low for the larger one to get under. Within a few more days they were sleeping together.
Oh, and a mirror with the little...