In case anyone else ever stumbles upon this question...I figured out how to solve my problem. Use a combination of nchoosek and combvec:
c1 = nchoosek(n1,k1);
c2 = nchoosek(n2,k2);
c = combvec(c1',c2')';
To order the rows in the same way as shown in the question, use:
csorted = sortrows(c);