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novahaiku, I have a flock of rescued drakes.
My most recent 2 rescues -- juvenile pekin drakes -- came at the beginning of April in sorry state as they had been dumped in a light industrial area with little water.
The two were clearly tightly bonded and stood with their necks wrapped round each other when they felt threatened. Also, they were non-stop chattering together as only clutch mates do. But the slightly smaller boy had lost his feathers round his left eye and down the left side of his neck.
I had them in a large dogcrate in my house quarantining and nursing them back to good health. Within a couple of days, it was obvious that despite the tight bonding, the slightly larger drake was bullying the other and pulling out his feathers. I was particularly concerned about the eye getting damaged, so I partitioned the dogcrate. The two had time out of the crate in a dogpen in my house with a kiddie pool. The bullying persisted when out of their crate. In May, when I came to integrate the now healthy boys with my own drakes, the bullying persisted -- even out when they were free ranging. In desperation, I swapped the bully with my son's older pekin drake. The bully settled in well with the girls -- who vied for his attention. The boy that had been bullied wasn't close to my own pekin drake nor my son's but they were not fighting much either [drakes do have little pecking order spats, that I leave for them to sort out themselves.] After 2 month's separation, at the end of August, my son's drake went home, and bully juvenile returned to me. He had moulted and regrown feathers damaged by cruel treatment before I got him, and had become a handsome adult drake. He was in see-but-no- touch, but his clutch mate was there by his pen all day and they were again chattering non-stop as only clutch mates do. So I let him out after only one day. He tried his bullying -- excluding the other from food and the kiddie pool -- but the other drake was having none of it. He wasn't going to be excluded from "his" kiddie pool or "his" food. He stood his ground!!! Interestingly, bully drake stopped trying to bully the other within a couple of days.
They have just been on a road trip with one of my muscovies and me. They are able to share a dogcrate now without any pecking and pulling head and neck feathers. Although, I had three dogcrates for them to sleep in, inside my friend's shed, they mainly chose to share a crate rather than go each into one [even my muscovy once got into one dogcrate with both of them -- ducks are quite daft!]
So this is just to illustrate that some drakes do bully others -- excluding them from food, drinking water, and wading pools. Mine was excessive and warranted separation for a while because of danger to the bullied duck's eye. Most drakes will sort differences out themselves. My other drakes -- 2 muscovy and a pekin had got pretty tatty by the beginning of September with loss of chest feathers and tail feathers, and beaten up flight feathers, resulting from normal pecking order behavior over the previous year.
Your drake's loss of feathers may be moulting or it may be the new drake pulling out the feathers like my three do. I leave my three to sort themselves out [although I only took one of them on the road trip, as he was scrapping with both the two I left at home.] By the beginning of October, all 5 have moulted and are looking handsome boys in their new feathery finery. As eyes and faces are not involved, I am leaving all 5 to sort out themselves now they are all back together [although I do put the hose on any two I see fighting while I am in the garden!]
I suspect yours will sort themselves out and not need separating. At the most they might benefit from sleeping in a separate compartment in your coop when you make it. My coop design is unsuitable for adding separate compartments, and so I add dogcrates when I need to separate warring boys. But I doubt that your two will need separate parts of the garden if you just keep boys and don't add the complication of female ducks.
Maybe, when you have built your coop, you could add two more boys, to give them something to think about, other than eachother!!