I have not balanced twin carbs, SU or Stromberg, since the 'seventies & only ever on straight 6 engines but the principle is very simple. Either buy a balancing kit, which I've never used, or remove the air-cleaner(s) & use a flexible tube (rubber/plastic etc) long enough for you to place one end close to the carb intake & the other end to your ear. With the engine idling place the tube partially across the intake, not into it & adjust the carbs individually until the 'hiss' sounds the same from both carbs. Check the throttle linkage opens both carbs together.