Is it secure to concatenate two 16-bit random numbers into a 32-bit one?
The Intel post which I think you mean was discussed in this question and as I wrote there, the limitation only applies in the case of trying to combine PRNG outputs into values larger than the seed entropy (two 256-bit values in their case). Also mentioned there: cryptographic mixing does not increase the entropy you have, so if concatenation is insecure, just mixing is as well.

So, whether you can concatenate two 16-bit values depends on how the library/hardware works:


If you are getting raw noise readings it is unlikely it has 16 bits of entropy per value. You would need to collect many and mix them into a single value if you wanted truly random values. And you would need to find out what sort of correlations the data has to determine the entropy rate.
More likely, there is already some mixing/whitening going on that tries to assure full entropy for the output values. If that is the case, you can combine them just fine.
If the library uses a PRNG to generate more output values than the input entropy from the hardware you again cannot combine two values and retain full entropy.