Since noone else has answered, I'll give it a go:

The proper way to use Holosideroi/Cataphracts would be to use them as the "hammer", when the enemy is already engaged with the "anvil": Your infantry.

A quite well-known Macedonian, I believe his last name was "the Great" or something like that, used this tactic with reasonable success with his Hetairoi/Companion cavalry (hammer) and phalangitai (anvil).