Computing two 32-bit results in parallel and mixing them at the end does NOT mean collision resistance is only as good as a 32-bit hash. For that, you need to compute ONE 32-bit result, then transform it into a 64-bit result.
Depends whether the two 32-bit hashes are correlated with each other. If there is no correlation then a pair of 32-bit hashes is no more likely to collide than a single 64-bit hash. But this is difficult to achieve, and you should not assume (for example) running the same algorithm twice with different initial states will produce uncorrelated hashes.
