
I would think that an 8 x 6 shed with as large a run as you have room for would be great. I wouldn't think you'd need separate sleeping quarters, dogs like company.
I do also want to say though that it's rarely a good idea to get two pups together. Many posters on here (including myself) who have vast experience of raising dogs have had problems with littermates or two dogs close together in age. It may be a good idea to get one pup just now while your old dog is around to help teach it manners, when that pup is 2 or 3 you can then think of getting another?