I just got a used Leva Dual and decided to descale it. While investigating a slight hiss from the vacuum valve I stumbled upon a technique that I'll definitely use again. Descaling process was as follows;

1) Open machine, ensure both boilers are depressurised, remove vacuum breaker valve from steam boiler and pressure gauge feed from brew boiler
2) Use a length of hose to drain both boilers. (Brake bleeding vacuum pump keeps hot water away from fingers and mouth and sucks out the bottom of the boiler)
3) Use funnel and hose to refill both boilers (to 80% full) with descaling solution
4) Replace vacuum valve and pressure gauge feed.
5) With descaling solution in the reservoir, turn the machine on and allow the steam boiler to auto fill then turn the pump on to fill the brew boiler (until water runs through the group head)
6) Allow the machine to reach operating temperature, run descaling solution through hot water tap, steam tap and group then turn off and allow to sit (length dependant on scale level)
7) Fill reservoir with fresh water, turn on machine and run some water through the group. Turn off.
8) Open depressurised boilers, drain, refill (80%) with fresh water, seal boilers.
9) Turn machine on, fill boilers (per step 5) and once at operating temperature run water through group, turn off, then depressurise steam boiler through both taps.
10) Repeat steps 8 & 9 but this time use some PTFE tape to seal the vacuum breaker valve.

A bit of a faff but seems easier than using vibration pump on and off. I would've done this on my old HX machine too as it took so many cycles to fully remove acid.