As the description states, the HD Flex Arm has a 3/8"-female thread in its base (same end as the 5/8" hex stud which slots into the SuperClamp) and the end has a 1/4" male tip. So simply put a 1/4" > 3/8" thread adapter on the tip, and this tip will then screw right into the base of the other HDFA.
Alernatively, if you don't need the snakiness of the HDFA, you could try the
Bogen extension pole. I've not used one, but from the looks of the pic and specs, I think you would just need to use the standard SuperClamp stud + whatever thread adapters you need to get a 5/8" tip on the stud, then screw the pole right on top.