Equivalent of union for rigid conduit installation?
What you're after, traditionally, is a three-piece coupling
The fitting you're after to join a pair of rigid conduits where you can't turn one or both conduits is called a three-piece conduit coupling.  As its name implies, it consists of three pieces: a doubly-threaded (inside and outside) chase nipple, a female threaded ferrule, and a female threaded hex coupling.  The chase nipple threads on one end, then the hex coupling is slipped over the other before the ferrule is threaded on.  Finally, the hex coupling is pulled over the ferrule and joins to the outside threads of the chase nipple, completing the joint as depicted in this video by electrical-fittings manufacturer/distributor Garvin Industries, who I have utterly no affiliation with.
There are other options, though
However, three-piece couplings aren't the only game in town.  There are set-screw and compression-type couplings available for rigid conduit (in addition to the EMT versions that folks usually know about).  Furthermore, there are also split couplings that are placed over the threaded ends you wish to join together and then bolted closed much like a pipe clamp.