I think the idea of having two, is so that one can be used as a reference.
However, I was just measuring a resistor with the outer coating removed, and didn't see much change between when I measured it in the room, oh!, just a minute...
I measured 130 ohms before I tried changing humidity, and around 122 ohms in water. (for sure 100%) and then with the resistor sandwiched between a couple of silicon bags (you know, the ones to keep electronics dry,) and now am reading 187 ohms. It is still drying out, now 192 ohms.
My conclusion is that this is do-able. I see my ohm meter is now reading 206 ohms. I pulled the resistor out of the silicon bags, and the ohm meter started to drop immediately, so it responds fairly quickly, but perhaps not as quickly as a thermistor.
I don't think it will be a linear change, but it would be a cheap way, and if you were planning on doing this a number of times, then the work to scale it would be reusable.
Something else: I just applied water to the resistor again, and my reading is 111 ohm, so some error in measurement. Perhaps the battery in my ohm meter is not in good shape, but I think this is a workable solution.