[pclmulqdq]

You wouldn't XOR-combine, you would hash them together. Something like:

SHA256(32B from HWRNG-A || 32B from HWRNG-B)

still guarantees you 32 bytes of entropy if either HWRNG-A or HWRNG-B is compromised, and if HWRNG-A and HWRNG-B are both partially compromised you also get 32 bytes of entropy. XOR has weird failure modes (eg if HWRNG-A and HWRNG-B are correlated).

[njtransit]

Is that a bit-wise OR? That seems like it would greatly reduce entropy. The probability of any given bit in A | B being 1 is 75%.

[ztorkelson]

It’s concatenation.