I am afraid there are no direct measures of multiple jobholders available in the American Community Survey or Decennial Censuses upon which IPUMS-USA is based. To my knowledge, no one at IPUMS has attempted to create such a measure using ACS or Census data, either. You could possibly create a new variable that attempts to guess whether a person is a multiple jobholder based on occupation and usual hours worked. The Current Population Survey does track numbers on multiple jobholders that could be used to check such a measure. Currently, IPUMS-CPS does not offer the multiple jobholder variable, but you can see this forum post for a method to merge the data from NBER onto an IPUMS-CPS extract.
I hope this helps.