As far as I understand most digital backs have two or more separate analog readout channels from the sensor.
"Most" is correct. In this post, I will point out specifically which do and which don't.
If you want an MFDB which does not (cannot) have this split-readout calibration problem, look for ones with Kodak 16.7MP or 22MP sensors...like the P20/+ and P25/+. All pixels are readout through the same, single corner amplifier.
The other Kodaks all have dual (18MP, 31MP, 39MP) or quad (40MP, 50MP) outputs, and as such are in principle
That said, the ones I've most commonly seen such problems posted about are the DBs with Dalsa sensors...like Kevin's Aptus in this thread. Every Dalsa DB sensor (right back to the 35mm-format 6MP and 11MP ones) has dual registers and quad outputs...which is why the Dalsa DB frame-rates have often pipped their Kodak counterparts...although the DB makers did not necessarily use all 4 outputs; using only 1 or 2 is possible. This trades off readout speed for calibration consistency.