HRHHID2 must be used in combination with HRHHID in order to identify households across samples. Since IPUMS and NBER files share a common sort order, you can perform a sequential merge in your stats package to merge HRHHID from the NBER basic monthly file onto the IPUMS file. That is, match the first NBER record to the first IPUMS record, the second to the second, etc for each monthly sample. You can then use the HRHHID and HRHHID2 identifiers as usual to match households across samples.
Additionally, IPUMS-CPS expects to release HRHHID for 2011-forward by the end of this summer.
Hope this helps.