You can get the desired result by running an aggregation:
FT.AGGREGATE cities_idx "*"
LOAD 3 @__key @fieldOne @fieldTwo
APPLY "format('%s:%s', @fieldOne, @fieldTwo)" AS combo
FILTER "@combo=='Foo:Bar'"
GROUPBY 1 @__key
This will return: search:cities:1
You can then do JSON.GET search:cities:1