Double de-clutching isn't needed in autos unless you have one that absolutely requires it to shift. These 1927s are some of last ones needing it.
Unless you are decelerating quickly, such as under maximum braking, blipping isn't needed.
Rev-matching works smoother:
1. adjust throttle to steady-state speed
2. squeeze-clutch, notice revs don't change if you did #1 correctly
3. shift down one gear
4. at same time,
increase revs by 500rpms
5. let out clutch
If you did #4 & #5 just right, revs won't change when you let out clutch and there won't be any jerking of bike.