Thanks to Discord user STLF for stumbling upon this detail! It turns out that Union Monsters were all printed with a Condition restricting the number of Union Monsters that could be equipped to a Monster. This Condition would cease to be printed, and would be retroactively removed from Union Monsters through erratas as of 2016 and the release of Structure Deck: Seto Kaiba [REF]. When playing Edison Format, remember that ALL Union Monsters have the following Condition:
"(A monster can only be equipped with 1 Union Monster at a time. If the equipped monster would be destroyed, destroy this card instead.)"
The "Edison Format Rules" page has been updated to reflect this discovery! For a list of EdisonFormat-legal Union Monsters, click HERE.