Goose is quite fatty. If it ends up dry it was vastly overcooked. Duck breasts (if skinned) are rather lean. The issue is more of the reverse: making sure the breasts don't overcook and/or cook evenly but just to the right point at the time the goose is done. I usually cook duck breast to medium rare - but this is on a grill or in a pan. They'd be rather hard to monitor if stuffed into a goose and the odds of uneven cooking are good.
When I stuff goose with duck I usually use either duck confit or a duck-based paté. In both cases I use either prunes or dried whole apricots, plumped well in boiling water, then stuff the prunes or apricots with the confit or paté, then stuff the goose with the stuffed fruit. Alternatively, you can chop or cube some duck breast and lightly sauté with onion or shallot and a few herbs, fold in some minced dried fruit (along with some fresh fruit like chopped apple or halved grapes), with a splash of wine and Cognac (or brandy), then use this for the stuffing (bind with a bit of bread if desired). Make sure the stuffing reaches safe temps during cooking (you can be assured of this by making the stuffing last and stuffing the goose while it is warm).
