A Jaeger has two legs and yet it is able to balance itself on them alone under different terrains including ocean floors.
This is explained (in excruciating detail) in the film's official novelisation. In short, the Jaegers are kitted out with automated balance units based on gyroscopic technology. Later Jaegers evidently have gravity manipulation technology whereas the older units make do with physical gyroscopes and jet thrusters:
Striker Eureka possesses a...
Burst propulsor and gravity capacitor system, combat-class balance enhancement
Crimson Typhoon has...
Enhanced balance systems and leg-integral Thrust Kickers
These were evidently added after a number of fatalities from falling:
A number of Rangers experienced battlefield fatality within unbreached Jaeger cranial Conn-Pods due to the impact of falling as their damaged Jaegers lost balance. The Mark III now has updated and enhanced gyroscopic stability as well as internal enhancements to the cranial framing.
Later we see that Gypsy Danger also has similar systems;
Use the gyroscope, Raleigh thought. Duh. Gipsy Danger had a number of balance systems, but it wasn’t designed for operating in the air. Raleigh swiped through a series of quick commands engaging the gyroscopes to keep them steady in midair. That took almost a full minute.
I don't think the humans piloting Jaegers were concerned with balancing the device. I think gyroscopic systems would be involved with such things in the same way a Segway maintains its own balance using gyroscopic technologies.
I think the pilots augmented the function of the device in CONJUNCTION with a number of supporting technologies.
Such things were not covered in any detail in the movie but it is probably safe to assume the approaches to cities used by the Kaiju and fought in by the Jaegers was well known enough to the pilots to not be a serious issue.
Harris, Tom. "How Segways Work" 03 December 2001. HowStuffWorks.com. http://science.howstuffworks.com/engineering/civil/ginger.htm 11 October 2013.
My impression is that the whole concept of 'drifting' is to provide a mechanism by which the control of the Jager is effectively an extension of the pilot's body so the pilots brain is effectively use as a control system, at least for the aspects which are a close analogy to human movement.
By extension it is reasonable to suppose that two, linked pilots are required in order to allow some spare capacity for operating additional systems like weapons and power management which aren't intuitive motor responses and also that the extra demands of controlling such a large machine, in addition to maintaining a sufficient sense of self to make tactical decisions is too much for a single person to cope with.
It's also a reasonable assumption that a machine of this size, even if it is closely modelled on a human body is sufficiently different that it needs some automate controls to be usable eg you might have a gyroscopic system which tries to keep it upright but with human pilot making the fine adjustments.
A good analogy is modern fighter aircraft where there sheer volume of information that a pilot needs to monitor and assimilate as well as the fine motor control and physical stress tolerance required means that there is a very real risk of the pilot becoming dangerously disorientated and unable to make decisions quickly enough.
The pilots use they natural sense of equilibrium to keep the machine standing.
Before deploying Gipsy Avenger, the pilots once received (quite ironic) instructions: just try to look nice and attempt not to tip over (my track is German so unsure about the wording). Imagine a battle tank driver hearing this - unlikely. The pilots are responsible for not tipping over. As Jaeger fights at the martial arts level, an automated system is unlikely to provide a reasonable assistance for equilibrium during combat - these movements are very complex.
It may be various systems available to make this easier or feel more natural.
There are numerous existing technologies that keep things balanced on non-level surfaces. This is just a much larger scale. Things like robots and the Segway already use this technology.
