[Zeke]

These things are made funky. IMHO, there should not be a hex shape on the side where the large hex is. It confuses the way you put the wrenches on to loosen the fitting.

As it looks from here, you need one wrench on the left side and one on the large hex. Using the righty tighty, lefty loosey axiom, pull the large nut down towards you while pushing the smaller hex upward.

You will then need a true pipe wrench to hold the pipe while removing the union half. Then cap with new pipe dope or teflon tape on clean threads.