That was the Speed Select Lever (SSL) that selected Nr above 100% (107% for take off) and had a knurled knob for Tq matching.
The only time you needed a third arm when flying without a qualified person in the left seat was just after lift off, when it was normal to match the torques, so if you had a technician in the left seat (for blade tracking etc) as a Maintenance Test Pilot (AKA Theatre Ground Runner) it was routine to brief them on how to match the torques using the SSL, or if it wasn't a big torque split, wait until you were airborne and do it yourself. The Lynx would be subject to vibration analysis using the "bonk", which was a very heavy, spring mounted weight attached to the top of the MR Head. Each bonk had a resonant frequency at which it provided the maximum counter effect to the Aircraft vibration, and during Vibe runs you would find the Nr % at which this occurred. This Nr value was then DymoTaped to the instrument panel and, in theory, the aircrew would set this value using the SSL when flying to minimize the vibration levels. Westlands and the Army spent a fortune on this solution, but it still relied on the pilot remembering to read the placard and set the value - which didn't always happen.
There was also some huge MoD Procurement level inquiry into why Lynx vibration levels were so bad (and very expensive in terms of maintenance) and after a few more million pounds the conclusion from Westland was the Army's Anti-tank Helicopter had excessive vibration because the Army had fitted it with Anti-Tank TOW Missile Booms! Apparently any requirements or specs against vibration levels were only warranted against the "slick" version of the Lynx, once you put role equipment on, it was your (Army's) fault. A classic procurement 'gotcha' and I'm sure we all learned from that...not.