I found the definition of 'two-six (2-6)' to be a little misleading, it seems to define the present meaning of the term whereas it actually pre-dates the RAF and has its origin in the RFC/RNAS. Aircraft of the day were very simple machines, no brakes, no steering and only a hardwood tailskid to provide stopping friction. Therefore the procedure for ground-handling these aircraft was to position one man at each lower wingtip (the 'two' of two-six) and three men along each side of the fuselage between the wing and tailplane (the 'six' of two-six). When ready, the person-in-charge of this ground handling party would call "two-six, up" and the fuselage would be lifted by the 'six' and the aircraft would be physically moved from one location to another with the 'two' ensuring the aircraft didn't rock too much and also that there was sufficient clearance for the wingtips - a practice still being followed today.
In my RAF time, the call 'two-six' usually meant everyone to help open/close the hangar doors or move something particularly heavy or awkward.