I know in various novels it has been said that some 'mechs with a second seat in the cockpit have used them to seat a gunner who fires the weapons (much like the backseater in larger tactical fighters today) while the front seater simply moves or pilots the 'mech.
And IIRC the Superheavy (over 100 ton) triple legged 'mechs have a dedicated gunner position aboard though I'm rather unfamiliar with those.
How much does having a second person control the 'mechs weapons increase its combat efficiency? Or are there other good reasons to have a second crewman aboard?
The Novels are talking about the Command Console as Firesprocket noted.
Basically the Backseater is a Tactical Officer that is in the mech but is doing Leadership functions & can grant initiative bonuses.
The Dedicated Gunner concept has to do w/ a technology that is no longer canon IIRC.
I want to say it was from MW2 Companion or Tactical Handbook or Solaris, but it was essentially the Dual-Cockpit system.
In that system it was a true 2 person cockpit where you split functions into Pilot & Gunner.
The benefits were bonuses (+1) to the Gunnery/Piloting rolls IIRC.
This is sort of how the Tripods do it now.
This system was lighter, only 1 ton extra, because it didn't duplicate all functions & if 1 of the 2 positions was critted then the remaining mechwarrior was at Negatives to both abilities IIRC since he/she could neither focus on the primary function & didn't have a full set of controls for the secondary function.