I am not certain which two data sets you are referring to in this question. Since, you’ve nested this question under the IPUMS USA category, it seems that you may be asking about merging US Census or ACS data with the NHIS data. Let me know if I am missing something.
With that said, this sort of merge is complicated due to several details. First, the sample of individuals is different between the US Census/ACS and the NHIS data. Therefore, merging at the individual level will not be possible. Second, in the public use NHIS data the lowest level geographical area is the REGION and in the US Census/ACS data the lowest level geographical area is the PUMA. Therefore, merging at a relatively low level geographical area will likely not satisfy your requirements for your research.