My experience has been that getting sufficient water flow (10-20L/m)
can be a problem even over short distances. Using 28mm piping as much
as possible and both Essex and Suffolk flanges in parallel from the HW
tank has been the best IME.

You may find this is because the hot water cylinder has poor connection with
the header tank. Ensure the feed to the header tank is at least 22mm and has
only flowed bends over a short distance and no bore reducing valves. If this
is done, you might not even need a flange at all, let alone two.

Christian.