A few differences listed here -
https://www.soundonsound.com/reviews/tascam-model-12
"Arguably the biggest ‘game changer’ is how you can deploy these processors. In the Model 24, the A‑D and D‑A converters were at a fixed point in the signal path, which limited how you could access the processing. For example, the compressors were always placed before the converters, which meant that you could record and mix live performances through them but couldn’t access or tweak them during playback. Similarly, the EQs always came after the converters, so you could mix through them but print the settings only in the stereo mix, not to the separate tracks in a multitrack recording. This has been ‘corrected’ in the Model 12: on each channel, you can choose via the Menu button and Multi Jog wheel to record the signal pre the compressor (ie. directly after the preamp, HPF and, on channels 1 and 2, the insert point), post the compressor but before the EQ, or post both the compressor and EQ. It’s a vast improvement, since the new routing options make possible so many different workflows."