is it true that if MOSFET are used in parallel, their thermal resistance (e.g. thJA) can be combined just like paralleled resistors?
Yes and no. The heat generated in each will be dissipated independently, at least as far as the case mounting point, so the total thJC will be halved. If they share a heatsink, then this becomes a common mode path in the way to ambient. If they are mounted far enough apart so that each does not affect the other's ambient temperature, then the total thJA will be halved as well.