2.75kg is a very large duck! Can your butcher guarantee the size or is the weight an approximation? I think even 2 x 2.75kg ducks is only just enough if you want to be sure that everyone gets as much as they want. People eat much larger servings at Christmas.,, If you end up with 2 slightly smaller ducks, it might not be enough.
Ducks shrink a lot as the fat melts off. You will need a roasting tin with a rack so your duck isn’t swimming in a pool of fat. Be careful taking it in and out of the oven!
I think the suggestion of confit duck or duck breasts or a 3 bird roast would make life a lot easier. I would at least buy some extra duck breasts or legs to cook with the 2 whole ducks. Cold duck is great in a salad or with potato galletes or bubble and squeak.
I think duck goes better with gratin dauphinoise than roast potatoes (although the duck fat makes great roasties), and orange/redcurrant/sour cherry sauce rather than gravy. If you want gravy, I would go for a slightly sweet red wine gravy. Braised red cabbage is perfect with duck. I also like some fresh steamed green vegetables to counterbalance the richness of the duck. Sprouts, chestnuts and pancetta also go well.
I would probably go with traditional Christmas sides (PiBs, sprouts, chestnuts, red cabbage, roast potatoes, roast parsnips) in case someone is especially looking forward to them and a gratin dauphinoise with both gravy and a fruit based sauce. No (not traditional) Yorkshire puddings though! If you want stuffing balls, a chestnut stuffing would go better with duck.