<<< native
[ EUnder
    False
    (EIdentifier "BASE")
    (EFraction
       NormalFrac
       (EIdentifier "first under script")
       (EIdentifier "second one"))
, ESymbol Rel "="
, EUnder
    False
    (EUnder
       False
       (EIdentifier "BASE")
       (EFraction
          NormalFrac
          (EIdentifier "first under script")
          (EIdentifier "second one")))
    (EPhantom (EIdentifier "fail"))
]
>>> eqn
BASE from {{first under script} over {second one}} = {BASE from {{first under script} over {second one}}} from {hphantom fail}