Hi , done this several times on some 3 stage Algi jacks I have on my route. Its not as bad as you think . Some times if you de pressurize the jacks all the way you don't need to hold the one underneath to unscrew the head above if you have the ones with the square sides for the jack followers like in the picture you can shock them with a lump hammer to get them started . There is a set screw on each head ..make sure you remove it before trying to unscrew them ! The times we have had problems with the jack ' spinning ' we made up some wooden braces for each piston with some all thread . I have a write up from Algi somewhere . Pm me your email address and I will send it to you . Be aware if you have problems with them going out of synch then you need to change the internal seals and check valves...and that's a whole different ball game !