chuck it in your drill and make one or two lengthwise cuts on the union just deep enough to cut through the wall of the union then use a screwdriver in the cuts to pry open the gaps. Rust resists shearing forces but is weak in resisting tensile forces and you will be putting a lot of tension on the rust bond.
Also, for reasons I don't understand, tightening the joint 1/8 turn may crack it loose.
It's the same technique for freeing painted-shut windows. No double-hung window is ever closed entirely so whacking it in the closing direction with a wood block buffer between the window and the hammer breaks the paint bond by moving the window edge frame a sixteenth or so.