No you can directly get the flat registered in your name from promoter as sale deed .
Or if promoter has already registered 2 flat on your name along uncle in joint ownership than you can make a partition deed you and your uncle on non judicial stamp paper and get it registered.
Yes you can sell undivided share without partition but in that can there can be a conflict so better to do partition and sale or your uncle can consent on your sale of your share so there won't be problem.
If uncle give consent that orally you have divided and he has taken one flat as his share and other share to you and can be made witness to sale deed.
In WB the stamp duty on family partition deed is 0.5% market value of property share.