When I have more space available than I need at full thrust I can reduce the thrust to the point that the space I need equals the space available.
Qantas was one of the leading lights in the reduced thrust development process. Wal Stack, at the time the boss ops engineer (and a thoroughly nice bloke as well as having a flying history), took the view that he would leave around a 1000ft accel stop pad for his crews, mum and the kids. I still think that that was a good strategy rather than going to the limiting case for the sake of a few extra kilos.
That's the balanced runway part.
Confusing two concepts I fear.
in which case you then need to advance the good engine(s) to max takeoff thrust.
Never the case. While the pilot retains the ability to advance thrust up to the relevant rating, reduced thrust is based on the philosophy that there is no need or requirement to do so. The caveat is that, should the pilot chose to advance the throttles, he/she should do so SLOWLY.
I also know a jet where advancing the thrust lever on the good engine could well cause me to loose shall we say "control on the ground".
Not only a problem with jets. I was involved in the investigation of a turboprop fatal in which our conclusion was that the pilots pushed up the throttles with an overshoot leading to a Vmca departure and the ensuing fireball ....
But, that is simply not the categorical case. (weight, performance, etc.)
Time for you to produce evidence to support such a statement ?
But, no competent performance engineer would permit that to happen
Said competent performance engineer is presuming that the pilot is NOT going to push up the throttles ...
Balanced Field Length
.. should always be optional other than for those aircraft which only have BFL AFM data (DC9 for instance, as I recall)