var another = from x in query
                          group x by x.index / 2 into groupped
                          select new
                          {
                              nums = groupped.Select(x => x.index + 1),
                              fruits = groupped.Select(x => x.fruit)
                          };

            var concat = from a in another
                         select new
                         {
                             indexes = a.nums.Aggregate("", (acc, x) => acc + (acc == "" ? "" : ",") + x),
                             fruits = a.fruits.Aggregate("", (acc, x) => acc + (acc == "" ? "" : ",") + x)
                         };