There are numerous significant electrical and physical differences between a stereo amp and a pair of "identical circuitry" monoblocks. Here are four:
One electrical difference is crosstalk (L signal getting into R, and vice versa). Crosstalk is the result of inadvertent "leaking" of signal from one channel into the other. Two physically separate amplifiers are going to have less crosstalk if all other things are "equal".
Another difference is power supply loading. When electrical demands presented by output devices in one channel create fluctuations in available power, performance of the other channel will be degraded. Physically separate amplifiers have more electrical distance between their separate power supplies, and again, all other things being equal, the separate amps perform better.
High currents in the chassis of a two-channel amplifier will modulate the electrical grounding within the stereo amp. Disturbances to electrical ground are often discussed as problems in precise amplification.
Even chassis vibration, propagating from one channel to another in a stereo amplifier, may cross-modulate signals and degrade performance. This is most often discussed with tubes -- microphonics.