Taking a quick look at the datasheet, you may need to select R3/R4 by doing some characterization. If the thermistors are of equal value, then you'd select the R3/R4 ratio to be equal to T1/T2 at 50C. They suggest that R3=10k and R4=15k. Since thermistors have a negative TC, at high temp T1 probably goes from a nominal value to maybe 50% lower resistance which moves the TEMP pin voltage above your preset voltage defined by r3/r4 on THI pin. It seems that a few things are not explained well in the datasheet, and some sentences are incomplete or poorly worded (as if English is not the author's first language). The term "voltage-on" is strange, for example.
For some reason, the MAX712 was a favored chip among ham radio guys for charging various cells, i think there was even a design in the ARRL handbook several years back.
Here's a citing from QSL.NET:
MAX712 NiMH Battery Charger
Here's a citing from QSL.NET:
MAX712 NiMH Battery Charger
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