Thermistor in parallel
Are there any downsides to putting two NTC thermistors in parallel
next to each other?

In terms of actually measuring the temperature, there should be no problem with putting multiple identical parts in parallel.

There is extra cost for more parts.  If you can it might be cheaper to just find a part that has a lower resistance to begin with (like 1K or 10K).

There is extra space taken up.

More parts to put down means more assembly labor.


There are also other options.

Buffering  the thermistor with a precision op-amp would probably get the best results.
If the sample rate is low, and the bias current on the ADC is low, then you could improve the measurement by putting a 1uF ceramic capacitor in parallel with the thermistor but near the ADC.  The capacitor would serve as a low impedance charge reservoir to feed the ADC each time it takes a sample.